Holiday Seafood Casserole

3 tablespoons butter
1 cup chopped onions
1 1/2 cups chopped celery
2 1/2 cups milk
6 tablespoons all-purpose flour
1 1/2 teaspoons butter
4 ounces Cheddar cheese, sliced
1/2 teaspoon salt
1/2 teaspoon black pepper
1/4 pound crabmeat
1/4 pound lobster meat
1/4 pound medium shrimp
1/4 pound scallops

Preheat oven to 350 degrees F (175 degrees C). Lightly grease a medium baking dish.

Melt 3 tablespoons butter in a skillet, and saut