HONEST, DOWN-HOME vegetable casseroles such as Sunday dinner scalloped potatoes and Aunt Sally's secret broccoli recipe have suffered reversals in recent years, as Aunt Sally discovered high technology and began besmirching perfectly good broccoli with onion soup mix and potato chip toppings. But with the current renaissance of good country cooking, these honest dishes are reemerging clean and imaginative once again.

Vegetable gratins have pleasant associations for many of us, even though we knew them by less glamorous names. Although the word connotes cheese, it really only signifies a dish with a nicely browned crust on top -- the result of bread crumbs and/or cheese spread generously and baked or run under the broiler.

The basic old-fashioned version consists of a cooked vegetable, a white sauce and a topping. But the variations are endless. Some gratins, notably potato, use uncooked vegetables and allow them to cook gently while the crust forms simultaneously. Others, like the gratin of endive that follows, use only heavy cream for the sauce and are baked at a higher temperature to form the crust. Still others, usually the lighter, more summery ones, rely only on the juices of the vegetables for moisture.

For these gratins and others, use a shallow, heavy baking dish of maximum width so that the greatest possible surface area is exposed to heat. This will allow for more crust and fewer fights over who's "pigging" it. Ingredients should be matched to the container in such a way that the dish is filled to the top, with just the slightest bit of space left for bubbling.

Other gratin pointers:

* Vegetables that have been parboiled should be drained well and patted dry before being combined with sauce.

* Oven temperatures can be flexible. If you have a roast in the oven at a lower temperature these gratins can go in beside it. Obviously they will need longer to cook to brown at lower temperatures.

* All these dishes profit from having 10 or 15 minutes to sit and compose themselves before being served. GRATIN OF POTATOES AND CAULIFLOWER (6 to 8 servings) Mornay sauce: 2 1/2 tablespoons butter 4 tablespoons flour 3 cups milk Salt, pepper and nutmeg to taste 3/4 cup grated gruyere or swiss cheese Vegetables: 1 small head cauliflower (about 4 cups florets) 4 medium potatoes (about 4 cups sliced) 1 tablespoon butter, plus a little for the dish 1/4 cup bread crumbs made from unsweetened white bread, preferably french or italian

To make the sauce, melt 2 1/2 tablespoons of butter in a heavy saucepan over medium heat. Add flour and cook, stirring, for 2 to 3 minutes. Add milk gradually, stirring constantly, until the sauce is thick and smooth. Season with salt, pepper, and a little grated nutmeg. Let this sauce simmer very slowly, stirring frequently, while you prepare vegetables. (Cheese will be added later.)

Break cauliflower into florets and chop roughly into pieces about 3/4-inch square. Peel potatoes and slice thin. Remove the sauce from heat and add 1/2 cup grated cheese. Stir until smooth. Taste for seasoning.

In a buttered gratin or other shallow baking dish of about 8-cup capacity, spread half the potatoes in an even layer. Sprinkle with a little salt and pepper, then add half the cauliflower, also in an even layer, seasoning as you go. Repeat layers. Sprinkle top with bread crumbs mixed with remaining 1/4 cup cheese. Dot top with 1 tablespoon butter. Bake in 375-degree oven for 45 minutes, or until vegetables are cooked and top is crusty and brown.

Let stand 10 or 15 minutes before serving. GRATIN OF EGGPLANT AND TOMATOES (4 servings)

Zucchini can replace the eggplant in this recipe, but tomato pieces should then be the size of zucchini pieces. 2 small or 1 large eggplant Salt and pepper 5 tablespoons (approximately) good olive oil 2 large tomatoes 2 cloves garlic, chopped fine 1 tablespoon chopped parsley or fresh basil 8 thin slices mozzarella cheese (or substitute 6 ounces grated swiss or 6 ounces grated parmesan cheese, or 3 ounces of each)

Peel eggplant and slice about 3/8-inch thick. Put eggplant slices in a colander, salt heavily, mixing salt and eggplant thoroughly, and leave to drain for 30 minutes or so. Rinse eggplant and dry well with paper towels.

In a large skillet, heat 3 tablespoons olive oil and add eggplant slices in a single layer. Saute' for 5 or 6 minutes on each side, adding more oil if necessary. You will probably have to do this in batches. Drain on paper towels.

While eggplant is saute'eing, peel and core tomatoes, then cut in half crosswise and squeeze gently to remove seeds and extra moisture. Slice about 3/8-inch thick.

Lightly oil a gratin dish or other shallow baking dish of about 6-cup capacity. Lay a slice of eggplant in bottom of dish, then a slice of tomato slightly overlapping it. Continue to overlap slices in rows or circles, until all are used. Sprinkle with garlic and parsley or basil as you go. Season with pepper, but be careful about salt as the eggplant may be salty enough already.

Top with mozzarella or other cheese, and drizzle about 1/2 tablespoon oil over the top.

Bake in a 425-degree oven until top is brown, about 15 minutes. GRATIN OF BROCCOLI WITH PANCETTA (4 generous servings)

Try this with brussels sprouts and cauliflower, too. 1 clove garlic, unpeeled 1 1/2 tablespoons butter, plus a little for the baking dish 2 tablespoons flour 1 1/2 cups milk Salt and pepper 1 bunch fresh broccoli (about 1 1/2 pounds) 3 ounces pancetta or bacon (pancetta is available in specialty stores) 1/3 cup whipping cream 3 tablespoons grated parmesan cheese, or a mix of parmesan and romano

3 tablespoons bread crumbs made from unsweetened white bread, preferably french or italian

Cut unpeeled garlic clove in half and rub cut edge all over the inside of a 6-cup gratin or other shallow baking dish. Let dish dry completely and then butter. Save garlic.

Make a white sauce by melting butter in heavy saucepan over medium heat. Add flour and cook, stirring, for 2 or 3 minutes. Add milk gradually, stirring, until the sauce is thick and smooth. Season with salt and pepper, and toss in the unpeeled garlic clove halves. Let the sauce simmer very gently while you prepare vegetables.

Cut off bottom half of broccoli stems and cut broccoli into long florets, with stems about the size of a little finger. Parboil in boiling, salted water for 4 to 5 minutes, or until the broccoli is just barely tender. Drain thoroughly and pat dry with paper towels. Dice pancetta and let it render its fat in a heavy pan over low heat (about 15 minutes). Don't let it burn or get too crisp. Remove with a slotted spoon and drain on paper towels.

Put half of broccoli in the bottom of your gratin dish, arranging it so it's even. Sprinkle with half pancetta. Pour half of white sauce over, straining to remove garlic. Then repeat layers. Pour heavy cream over all, then sprinkle top with crumbs and cheese.

Bake in a 425-degree oven until top is brown, about 20 minutes. Let stand a few minutes before serving. ENDIVES AU GRATIN (4 servings)

This is a rich dish to serve with simple roasts or chops. The same technique can be used with leeks, cabbage, quartered fennel bulbs, baby onions, swiss chard or spinach. 8 large, firm endives 2 tablespoons butter, plus a little for the gratin dish Salt and pepper 1/3 cup whipping cream 1/3 cup grated swiss or gruyere cheese 3 tablespoons bread crumbs made from unsweetened white bread, preferably french or italian

Wash and trim endives, then cut in half lengthwise. Be careful not to trim too much of stem end or leaves will detach.

In a heavy-bottomed casserole dish with a top, melt butter over medium heat. Add endives, turning over to coat with butter. Season with salt and pepper. Let endives braise in butter over low heat, covered, stirring frequently, until they are tender, about 20 minutes. Endives should be just beginning to brown.

Butter a gratin or other shallow baking dish just large enough to hold endives in one cozy layer.

Pour cream over the top, sprinkle with crumbs and cheese, and bake in a 425-degree oven until top is brown and cream has thickened into a rich sauce. Let stand a few minutes before serving.