Cook celery, mushrooms and green pepper in 2 tbsp. butter until tender. Melt remaining 4 tbsp. of butter over a low heat. Blend in flour and 1 teaspoon salt. Gradually add milk, cook and stir until thickened.
Combine scallops, vegetables, and sauce. Place in a greased casserole dish. Top with bread crumbs and sprinkle with grated cheese.
Bake in 350º F. oven for 20 to 30 minutes.