Add butter, flour, and milk to a sauce pan and heat over medium heat, while stirring, until it's well blended and thickens. Allow to cool slightly.
Transfer your white sauce to a large bowl and mix in the crab meat, beaten egg, pimiento, mayonnaise, dry mustard, salt & pepper.
Place mixture into 4 crab shells, scallop shells, or individual baking dishes.
Sprinkle equal amounts of bread crumbs, Old Bay Seasoning, & thin slices of butter to cover the tops of each serving.
Bake in 350 degree fahrenheit oven for approximately 15 minutes. Then turn your oven on broil for 3 to 4 minutes, while keeping a close eye on them, to brown the tops.