Butter a shallow casserole (1 1/2" deep). Cover bottom with 1/2 the flounder. Mix all ingredients for crab filling and place it on top of fish. Cover with remaining fish. Pour Mornay sauce over fish and bake at 375 for 30 minutes or until fish is tender.
Melt butter for sauce; blend in flour, and slowly add chicken stock and cream, stirring constantly. Bring to a boil; lower heat and simmer 5 minutes. Add cheese; cook until melted.