Preheat oven to 375°F. In a skillet over medium heat, melt the butter. Stir in the onions, mushrooms, and tomato, and sauté until the onions are translucent, 3 to 5 minutes. In a lightly oiled ovenproof casserole dish large enough to hold all the fillets, lay in the fish and spoon the stuffing on half of each. Fold over or gently roll up. Sprinkle with breadcrumbs and grated cheese. Bake for 25 minutes. Serve with sliced tomatoes topped with fresh chopped basil.