Crumble crackers between waxed paper with a rolling pin. Reserve just enough crumbs and cheese to top the casserole.
Melt 4 tablespoons of the butter in a large heavy skillet. Add onion and saute for about 5 minutes or until golden. Slowly add flour, stirring constantly over low heat. When flour is blended, gradually add hot milk (scalded) and blend with a whip. Continue stirring over low heat until the sauce begins to thicken.
Add salt, pepper, and sherry and continue stirring in a bowl.
Mix crab meat, sauce and the extra cracker crumbs and cheese. Place in a lightly greased baking dish. Sprinkle reserved cracker crumbs and cheese on the top of the casserole. Dot the top with the remaining 4 tablespoons butter.
Bake uncovered for about 15 minutes or until the top is golden brown.