Preheat oven to 350°F.
Heat butter in a large skillet. Add onion, celery, and mushrooms. Saute for 5 minutes.
Stir in breadcrumbs and salt.
Coat a large baking dish with cooking spray. Place fish fillets in prepared baking dish.
Sprinkle fish with lemon juice.
Spread breading over fish and top with sliced tomatoes.
Bake uncovered for 25 minutes or until fish is done.