Sauté carrots, celery, onions and mushrooms in 2 tablespoons butter until soft. Stir in bread crumbs and 1/4 teaspoon salt. Sprinkle remaining salt over flounder.

Place half in a single layer in a greased 10x8x2-inch baking dish. Spoon vegetable mixture over flounder; layer rest of fillets on top. Dot remaining butter over fish; sprinkle lemon juice on top. Cover.

Bake at 350°F for 30 minutes or until flounder flakes easily. Serve from baking dish.

Makes 4 servings.