Bring water to a rapid boil in a heavy four quart saucepan. Add 1 1/2 teaspoons of salt. Gradually add the shells, making sure that the water continues to boil. Cook the shells uncovered until they are tender but still quite firm, stirring occasionally. With a slotted spoon, remove the shells from the water and drain them well. Cover to prevent them from drying out.

Melt butter in a medium saucepan. Add the mushrooms and sauté for about 5 minutes. Remove mushrooms and set them set aside.

Stir the flour into the butter. Cook over low heat until bubbly, stirring constantly. Continuing to stir, blend the milk, cream, 1/4 teaspoon salt and pepper into the flour mixture. Cook until the mixture thickens, about five minutes. Remove from the heat. Blend 1/2 cup of the hot sauce into the beaten egg yolks. Gradually add the egg yolk mixture to the remaining sauce. Stir in the sherry and Worcestershire sauce.

In a medium bowl, mix together the crab, shrimp, sautéed mushrooms, pimento(if using) and 3/4 cup of the sauce. Stuff the pasta shells with the seafood mixture. Pour most of the remaining sauce into an 11" x 7" baking dish and arrange the filled shells on top of the sauce, reserving the rest for the top. Spoon a little of the sauce over each shell and sprinkle with Parmesan cheese.

Bake in a preheated 350°F oven for 20 minutes.