I found this recipe, from Chef Maurice at the Pfister. My mother wrote down, and added it to her recipe file box, that she eventually gave to me.
Pfister White House Sandwich
4 slices bread
16 oz sliced turkey
8 oz sliced fresh mushrooms, sautéed but not browned
1/4 c chopped onions
1/2 cup butter
1/2 cup flour
2 cups cold milk
2 cups chicken stock
salt and white pepper to taste
3 egg yolks
1/2 cup whipping cream
1/2 cup grated Swiss or Parmesan cheese
Directions for making the sauce:
In a large saucepan, sauce onions in butter but do not brown.
Add flour and sit well over low heat. Do not let it brown.
Add cold milk and mix till smoother. Add brother and bring to a boil, stirring constantly, then simmer 10 minutes. Add salt and white pepper to taste.
To make the sandwich:
Preheat oven to 325 degrees. Place 1 slice of toast in each of 4 buttered individual casseroles and cover with about 1/4 cup of sauce. Divide turkey and mushrooms among the four sandwiches. Top with more sauce and sprinkle with cheese and bake 8-10 minutes or till light brown. Serves 4.