Preheat the oven to 375°F. Melt the butter in a large saucepan. Add the onion, celery, and mushrooms; saute for 5 minutes. Stir in the breadcrumbs, salt, pepper and tarragon; set aside.
Grease a large, shallow baking dish. Arrange the fish fillets in the dish and sprinkle with the lemon or lime juice. Spread the reserved breadcrumb mixture over the fish, and cover with the tomato slices. Bake uncovered for 35 to 40 minutes.
Makes 6 servings.