Thaw crab, if frozen. (Ask your butcher to crack fresh crab legs for you. Otherwise, use kitchen shears to crack them open.) Remove crabmeat from shells and cut up. Set aside.
Bake patty shells according to package directions.
In a medium saucepan cook mushrooms, leek or green onion, and garlic in margarine or butter until tender but not brown. Stir in flour and white pepper. Add half-and-half, light cream, or milk all at once. Cook and stir until mixture is thickened and bubbly; cook and stir for 1 minute more. Reduce heat.
Add natural Swiss cheese and process Swiss cheese to cream mixture; stir until melted. Stir in crabmeat and dry sherry. Heat through; do not boil. Serve immediately in patty shells. Garnish with fresh chives and thyme, if desired. Makes 6 servings.