Remove stems from mushrooms; set caps aside. Chop stems. Saute chopped stems with onion in oil in a small skillet over moderate heat, until onion is tender. Add zucchini and carrot. Continue sauteing 3 minutes longer, stirring often. Remove from heat. stir in parsley. Fill mushroom caps with vegetable mixture. Place, stuffing-side up, in shallow baking dish. Bake in moderate oven (350°) 20 minutes. Pour ITALIAN MARINADE over baked mushrooms. Cover and refrigerate at least 4 hours, or overnight. Makes 6 servings. ITALIAN MARINADE 2 tablespoons freshly squeezed lemon juice 1 medium clove garlic, minced, 1/2 teaspoon leaf thyme, crumbled, 1/2 teaspoon salt 1/4 teaspoon paprika 1/4 cup olive oil Combine all but olive oil in a small bowl. Slowly beat in olive oil in a stream, using a wire whip; until mixture is emulsified. Beat again just before using. Makes about 1/3 cup.