WE MAY poach an occasional egg for breakfast, or a whole salmon for a special party. But when it comes to Sunday dinner, we rarely think of poaching the meat. Like stewing, this cooking method lends itself well to tough cuts, turning them into tender, juicy dishes. And even delicate meats take nicely to poaching.

A list of favorite poached dishes from around the world would include some startlingly different foods. Consider the New England boiled dinner (which is poached and not boiled), corned beef and cabbage, chicken gently poached and decorated with aspic, duck filled with a smooth forcemeat of veal and pistachios, pears poached in red wine or cloud-like quenelles made of pure'ed sole and heavy cream.. Even tenderloin may be poached in a strong beef broth just long enough to leave it rare in the center yet moist and flavorful on the outside.

Poaching is defined as simmering food gently until it is cooked. Possibly the most difficult part of the process is keeping the liquid just at the simmer and believing that the food will cook. Liquid is simmering when the surface just begins to quiver but no bubbles appear. When bubbles appear over most of the surface, the liquid is boiling. Water simmers between 175 and 185 degrees; it boils at 212 degrees. This small variation in temperature means the difference between tough, dry meat and tender, moist meat.

Tough cuts will become tender when poached because the long slow cooking dissolves the connective tissue without leaching out the fat and juices. Boiling, on the other hand, hardens the connective tissue and drives the juices out, destroying the texture of the meat. Candidates for Poaching

The best cuts of meat to poach include very firm, well-worked muscles of large animals (such as chuck, brisket, flank, breast and shoulder of lamb or beef), and the very tender meats (such as tenderloin) in additon to fish, shellfish and poultry. Of course, eggs and fruit may also be poached.

Meat with large sections of fat are not a good choice since the fat becomes gelatinous during the long cooking and unattractive in appearance and texture.

Poaching liquid varies with the food being cooked. Sometimes plain water is used; or water flavored with a slice of onion, carrot and salt. Fish and shellfish take well to "court bouillon," a combination of water, wine vinegar, herbs and vegetables. Most of these liquids add only a little flavor to the food, and the resulting broth is too weak to be used as soup. But when there is some flavor in the liquid, allow the food to cool in it by placing the entire pot in a sink of cold water so that the cooking stops.

Unlike the stewed and braised dishes, poached meats are never browned before cooking. Because of this, meat dishes will appear pale. This is often disguised under a layer of aspic or sauce. Poaching Equipment While most poaching is done on the stovetop, inventive cooks have used a variety of appliances for poaching. I've read stories of fish poached in the dishwasher and chicken poached on the engine of a car traveling down the highway. And old war movies often include a shot of soldiers poaching a purloined chicken in their helmets. In fact, Army slang for that piece of equipment is "steel pot."

Most poaching, however, is done in conventional cookware -- stockpots, dutch ovens and other large pots. Because of its shape, fish presents a special problem. To accommodate the varying size of different fish, special poachers are made in lengths from 14 inches up to 36 inches. The longer ones sit over two or more burners. They include a perforated platform for raising and lowering the fish -- a considerable help once the fish is cooked and needs to be removed, since the cooked fish will break apart easily. I recommend buying the largest fish poacher that will fit on your stove and in your kitchen cabinets. This gives more flexibility in the size fish you can cook. Nothing is so disheartening as coming home with a beautiful, whole 8-pound salmon that won't fit into your 18-inch poacher.

The only other equipment you might need is a flame tamer. This is a metal disc that sits on top of your electric or gas burner, diffusing the heat and spreading it evenly. It helps in controlling the heat.

Meat and fish are usually wrapped in cheesecloth or muslin before poaching, for several reasons. The cheesecloth protects the skin on fish or poultry and maintains its shape. It also provides a convenient handle for raising or lowering the food into the poaching liquid.

When meat or fish are heated in the poaching liquid, some of the protein is drawn out. This forms a scum that rises to the top of the liquid, and it can be removed using a skimmer or slotted spoon.

Most poaching begins with the food immersed in a cold liquid which is slowly heated to a simmer. The cooking time varies depending on the cut of meat and its size. Doneness is determined by pressing with a finger to see how firm the meat has become, or by piercing with a skewer and observing whether the juices are clear. Pink juices in meat indicate that the meat is still rare. Most recipes will give their own test for doneness.

Some exceptions to the rule of starting in cold water are eggs, fish fillets and anything that is made of a soft dough like quenelles or dumplings. These are all dropped into simmering water.

For best results in poaching, follow these steps:

* Choose either a tough muscular piece of meat or a tender food that needs gentle cooking.

* Make the court bouillon or seasoned water that the food is to cook in. Your recipe will give you the directions. If vegetables are included and cooking time is brief (as for fish), cook the bouillon for 30 minutes and then cool to room temperature.

* Wrap the food in cheesecloth or muslin to hold the shape and protect it.

* Place the item to be cooked in a pot large enough to hold it with just a little extra room for liquid. Cover with cold water or the bouillon and bring the liquid slowly to a simmer.

* Skim off any scum that rises to the top.

* Adjust the flame so that the surface of the liquid moves but no bubbles appear. If you have an instant-reading thermometer, test the temperature, which should be between 175 and 185 degrees. Cover with a tight lid, except when cooking eggs, quenelles or dumplings.

* When done, remove gently or allow to cool in the liquid by placing the whole pan in a sinkful of cold water to stop the cooking. POACHED CHICKEN 3- to 4-pound chicken 1 tablespoon salt 1 teaspoon peppercorns Sprig of parsley 1 bay leaf Sprig fresh thyme

Wash the chicken. Place the chicken in a deep pot and cover with water. Add the salt, and a bouquet garni made by tying the seasonings in a small square of cheesecloth. Turn the heat to medium and slowly bring the liquid to a simmer. When the first bubbles appear, turn the heat down so that the surface of the water quivers. Cook for 1 hour. Lift the chicken from the liquid and prick the inside of the thigh to see whether the juices are still pink. If they are, continue poaching for another 10 to 15 minutes. When done, remove the chicken from the liquid and proceed as your recipe directs. I like using this poached chicken for salads and casseroles such as chicken divan. BLANQUETTE OF LAMB (6 servings) 4 pounds lamb shoulder, cut in 2-inch cubes 2 cloves garlic, minced 1 large onion, chopped in large pieces 1 pound carrots, peeled and cut in thick slices Salt and pepper to taste 2 tablespoons butter 2 tablespoons flour Freshly grated nutmeg Pinch of thyme 1 teapoon minced parsley 1 teaspoon minced chives 3 egg yolks 1/2 cup heavy cream

Place the lamb cubes, garlic, onion, carrots, salt and pepper in a deep pan and cover with cold water. Bring liquid to a simmer. Adjust the heat so that the liquid just moves. Cover and cook for 1 hour and 15 minutes. Pour the contents of the pan into a colander, saving the liquid. Keep the lamb and vegetables warm. In another large saucepan, heat the butter until it bubbles then add the flour. Cook for 1 minute, whisking. Add the nutmeg and thyme. Whisk in 2 cups of the reserved liquid and cook for 5 minutes. Add parsley and chives. Just before serving, mix the egg yolks with the cream and whisk into the sauce. Adjust the seasoning with salt and pepper. Reheat the meat and vegetables if necessary in the reserved cooking liquid then drain the meat and add it to the sauce. Serve with rice. POACHED SALMON (8 servings) 6- to 8-pound salmon, head and tail intact Cheesecloth and string 2 cups white wine 1 cup white wine vinegar 1 carrot, sliced 1 onion, sliced 3 sprigs parsley 1 bay leaf 1 rib celery, sliced 1 lemon, sliced 1 teaspoon dried dill weed Sprigs parsley and small piece of olive for serving Beurre blanc sauce for serving (recipe follows)

Wash the fish. Remove the gills, using scissors to clip them out. Lay the fish on the table and measure its thickness very accurately at the thickest part (it should be about 2 1/2 inches). Place the fish in the poacher and add enough water to cover. Remove the fish and set aside. (This step assures that you will make enough court bouillon.) Add all of the remaining ingredients to the poacher. Bring to a boil and simmer for 30 minutes. Cool the liquid and strain, discarding the vegetables and seasonings. Wrap the fish in muslin or cheesecloth and tie the ends. Place the fish on the poacher rack. Lower into the poacher. Cover with court bouillon. Place over a medium flame and bring up to a simmer. When liquid reaches the simmer, cook the fish for 10 minutes per measured inch (25 minutes if it measured 2 1/2 inches thick). To test for doneness, use an instant-reading thermometer. The internal temperature of the fish should be 140 degrees. Drain and place on a long serving platter. Garnish with sprigs of parsley. Replace the eye, if necessary, with a piece of olive. Serve with beurre blance sauce on the side. BEURRE BLANC SAUCE 1/4 cup minced shallots 1/4 cup white wine 1/4 cup strained poaching liquid used for salmon 1/4 cup white wine vinegar Salt and pepper 1/2 pound unsalted butter 2 tablespoons heavy cream

Place shallots, wine, poaching liquid, vinegar and salt and pepper in a heavy-bottomed saucepan. Reduce until only 2 tablespoons liquid remain. Cut butter into 1/2-inch cubes and whisk into the liquid in saucepan bit by bit. When all butter is added and the sauce is thick and creamy add 2 tablespoons heavy cream and adjust seasoning.