Heat the oven to 350° F. Rinse scallops and pat dry with paper towels. Sprinkle with ½ teaspoon salt. In a medium skillet, melt 3 tablespoons of butter. Add celery, mushrooms and green pepper and cook until tender.
In a separate medium saucepan, melt 4 tablespoons of butter over low heat. Blend in flour, ¾ teaspoon of salt and black pepper. Add milk gradually and stir until thickened.
Combine scallops, vegetables and sauce. Pour into greased 1 ½-quart casserole. Combine bread crumbs and cheese and sprinkle over top. Bake for about 30 minutes or until golden brown and bubbly.