Besides having one of the grandest dining rooms in Washington - whatever is not covered with red leather or velvet is covered with gold - Rive Gauche is capable of some of the grandest food served in Washington. Their young French chef is an innovator, and his scallops steamed with vegetables, moistened with a tart butter sauce, are an exquisite example of the new wave of French cooking. The best appetizers are the most elegant (fresh foie grass) and the most homely (potage St. Germain). Forget the onion soup. While salads are in general very fine like most French restaurants', Rive Gauche charges top prices ($7.50 for two) for Caesar salad, but so disdains it as to use packaged grated cheese on it. Otherwise, ingredients are top quality, and the cooking is often inspired. Mistakes are made - oversalted Nantua sauce, too-dry lobster in a $38 lobster souffle for two. But these are aberrations. Sometimes there are delicacies such as imported fresh baby string beans, or fresh fish flown in from France, and the daily specials not only reflect what is best in the markets, but what new ideas the chef is trying. Dessert, for instance, can be an intensely perfumed sherbet of the season, made daily in the kitchen. The wine list is impressive, but priced so that you need a sense of abandon to take advantage of its breadth. But if you knew the old Rive Gauche, today it is a comparative breath of fresh air, not only because of its imaginative chef, but because the imperiousness is gone, and the polished staff serves everyone, from tourist to celebrity, with admirable competence.