Cheddar Seafood Casserole

7 tablespoons butter or margarine
4 tablespoons all-purpose flour
1 1/2 cups milk
1/2 teaspoon salt
1/4 teaspoon thyme leaves
1/8 teaspoon each pepper and dry mustard
2-1/2 cups shredded medium or sharp Cheddar cheese
2 egg yolks, slightly beaten
1/2 pound mushrooms, sliced
1 jar (10 oz.) Pacific oysters, cut in bite-sized pieces
1 pound crab meat
1 pound medium-sized shrimp (about 32), cooked, shelled, and deveined
1 tablespoon lemon juice

In a 3-quart saucepan, melt 4 tablespoons of the butter; stir in the flour and cook until bubbly. Remove from heat and gradually add the milk; cook, stirring until thickened.

Add the salt, thyme, pepper, mustard, and 2 cups of the cheese; stir until cheese is melted. Add the egg yolks and cook over low heat 3 minutes longer; set aside.

Meanwhile, melt the remaining 3 tablespoons butter in a wide frying pan over medium heat; add the mushrooms and saute until lightly browned and the liquid is evaporated.

Add mushrooms and butter to the cheese sauce. To the frying pan, add oysters and their liquid; simmer until the edges begin to curl. Drain off liquid and stir oysters into sauce, along with the crab meat, shrimp, and lemon juice.

Turn mixture into a 2-quart casserole; sprinkle remaining cheese evenly over top. Cover and chill if made ahead.

Before serving, bake, uncovered, in a 375 F oven until cheese is melted and mixture is heated through, 20 to 25 minutes (30 minutes if refrigerated).