Seafood Casserole

2 lbs fish fillets, cooked
1 cup shrimp
1/2 lbs scallops
1/2 lb crab
5 Tbsp butter
6-1/2 Tbsp flour
1 tsp dry mustard
1 tsp salt
4 cups milk
2 cups cheddar cheese

Flake cooked fish, remove bones. Melt butter, blend in flour, mustard and salt; cook until bubbly. Add milk gradually. Cook and stir until thickened and smooth. Add grated cheese and stir utnil cheese is melted. Combine sauce and fish. Put fish mixture into greased casserole dish. Bake at 350 for 30 to 35 minutes.