Fish in Vegetable Sauce

Ingredients

• 2 tablespoons butter or margarine
• 1 small onion, cut into thin rings
• 1/4 pound mushrooms, sliced
• 1 medium-size tomato, seeded and chopped
• 1/4 teaspoon oregano leaves
• 1/3 cup tomato-based chili sauce
• 1 pound red snapper or lingcod fillets, about 1/2-inch thick and cut into serving-size pieces
• 1/3 cup grated Parmesan cheese