Alexander Sauce

6 tablespoons unsalted butter, divided
1/2 cup chopped sweet type or yellow onion
1 1/2 tablespoons flour
1/2 cup clam juice
2 cups whipping cream
1/2 teaspoon salt
1/4 teaspoon white pepper
1 cup uncooked small shrimp
5 ounces white lump crabmeat
Dash of cayenne pepper (optional)

Melt 3 tablespoons of the butter in a large saucepan. Add the onion and
cook until soft. Stir in the flour and cook about 2 minutes, stirring
constantly. Slowly pour in the clam juice and cream. Season with salt, pepper and
cayenne. Cook, stirring, about 3 minutes.
In another saucepan, melt the remaining 3 tablespoons butter. Add the
shrimp and cook about 2 minutes. Add the crabmeat and cook about 1 minute. Stir
into cream mixture.