There was a time when no self-respecting southern hostess would seat guests at her table without serving at least one congealed salad. That wasn't so long ago, and congealed salads aren't so far removed from our minds as to be forgotten.

So what happened to them? Well, we've turned them into cold mousses that have a cool refreshing texture somewhere between the old aspics and a savory Bavarian cream. We have also given them a new life by making them in flavors that don't come out of Jell-O packages. The three here are mustard, cucumber and roquefort.

Any of the mousses can be made in a six-cup mold of any shape. Use a ring mold if you wish to fill the center. The mustard ring could be filled with cole slaw, the cucumber mousse with curried chicken or turkey salad, and the roquefort mousse with a seafood salad. Chilled in a solid mold, try the mustard mousse with roast chicken or game; the cucumber mousse with poached, grilled or baked fish, or with spicy and curried dishes; and the roquefort mousse with grilled steaks, roast lamb or shrimp. MUSTARD RING (8 servings) 1/3 cup sugar 3 tablespoons dry mustard 1 tablespoon each dijon mustard and a coarsely textured mustard, or 2 tablespoons dijon 1 teaspoon salt 1 tablespoon (1 envelope) unflavored gelatin 1/4 cup cold water 3/4 cup red wine vinegar 4 eggs, lightly beaten 1 cup whipping cream, beaten stiff

Combine the sugar, mustards and salt. Soften the gelatin in the 1/4 cup cold water, then melt by stirring over a hot water bath. Combine the vinegar, eggs, mustard mixture and melted gelatin and stir over medium-low heat until thickened like a custard. Cool, then fold in the whipped cream and pour into a 6-cup ring mold that has been rinsed in cold water. Refrigerate overnight.

Unmold onto a serving plate. COLD CUCUMBER MOUSSE (8 servings) 1/2 cup cold water 2 tablespoons (2 envelopes) unflavored gelatin 1 cup boiling water Juice of 1/2 large lemon 1/2 cup dry white wine 1 teaspoon salt (optional) 1/4 teaspoon freshly ground black pepper 2 cups peeled, seeded and grated cucumber (4-5 cucumbers) 1/2 cup chopped peeled celery 2 tablespoons each chopped fresh parsley and dill 1 cup whipping cream, beaten stiff

Soften gelatin in water, then stir into boiling water until dissolved. Add lemon juice, wine, optional salt, and pepper. Refrigerate until thick and syrupy but not set.

Place the cucumber, a handful at a time, in a kitchen towel and squeeze hard to remove the excess water. Combine with the celery and herbs and stir into the gelatin mixture. Fold in the whipped cream. Pour into a 6-cup mold that has been rinsed in cold water and refrigerate until set, about 4 hours. HELEN CORBITT'S ROQUEFORT MOUSSE (8 servings) 1/4 cup freshly squeezed lemon juice 3 tablespoons cognac 1 tablespoon (1 envelope) unflavored gelatin 1 cup boiling water 3/8 pound roquefort cheese, at room temperature 1/4 cup finely chopped parsley 1/4 cup finely chopped chives 1 tablespoon finely chopped capers About 1/2 teaspoon salt, optional Generous half teaspoon coarsely ground black pepper 1 cup whipping cream, beaten stiff

Combine the lemon juice and cognac and add the gelatin. When gelatin has softened, stir in the boiling water, mixing until the gelatin is dissolved. Break off a third of the cheese and set aside. Mash the rest of the cheese in a mixing bowl using the back of a spoon.

Combine the cheese, parsley, chives, capers, optional salt, and pepper, then add the gelatin mixture and stir together. Chill in the refrigator for 15 or 20 minutes until the mixture begins to thicken. Meanwhile, break the remaining cheese into small pieces. Stir into the chilled mousse, then fold in the whipped cream.

Pour into a 6-cup mold that has been rinsed in cold water. Refrigerate until set, at least 6 hours.