Right up front we will admit that the best way to prepare fish is the plainest: rockfish cooked on the grill with a squeeze of lemon; bluefish simply broiled, with the merest dusting of paprika if you must; salmon poached or sole lightly saute'ed.

That said, let's get on to what do you do when you are tired of plain fish, or when you have children or a spouse who claim to dislike fish. You cover it with something they do like, so they will be ready for a second bite before it has even registered that it is fish they are eating.

Here's a way. Just call it "a casserole" and set it on the table. Put off further explanations until after the first taste. All it takes is a trip through the express lane, preferably at a market with a fish counter that will fillet your choice to order. All you need on hand at home is a little butter or oil for greasing the baking dish and seasoning the vegetable. Choose any fish you like, and try it with a different one the next time.

EXPRESS LANE LIST: onions, fish fillets, mayonnaise, worcestershire sauce, lemons, parmesan, broccoli or other vegetable. BETTY OWSLEY'S PIQUANT FISH (6 to 7 servings) Butter or oil for baking dish 4 medium onions, sliced and separated into rings 2 pounds fish fillets, any kind 3/4 cup mayonnaise 2 teaspoons worcestershire sauce 2 tablespoons lemon juice 1/3 cup grated parmesan

Butter a baking dish wide enough to hold the fillets in one layer. Boil onions in water to cover for 4 minutes or until transparent, drain, and spread the onions evenly on the bottom of the baking dish. Place fillets on top of onions. Combine mayonnaise with worcestershire and lemon juice and spread evenly over fish. Sprinkle cheese over all. Bake at 350 degrees for 35 minutes or until fish is opaque. Serve immediately with steamed broccoli or other green vegetable.