Preheat the oven to 375℉
Grease a casserole dish (1.5 pint).
Season the scallops with a touch of salt.
Prepare the bread crumbs by adding 2 tbsp of melted butter to the bread crumbs.
In a skillet, melt 2 tbsp of butter. Add onion, celery, green pepper and mushrooms. Cook on low heat for about 10 minutes or until the vegetables are partially cooked (still crunchy). Set aside.
In medium-large saucepan, mix 1/4 cup of melted butter, the flour and the salt. Cook on low heat, constantly stirring, until it thickens. Add the vegetables and scallops to this sauce, mix gently.
Pour the mixture into a casserole dish.
Cover with the buttered bread crumbs, sprinkle with grated cheese.
Cook uncovered for about 20 minutes or until the sauce begins to bubble and the bread crumbs are golden.