Cook the pasta according to the package directions, al dente; drain well and set aside. After you’ve started with the pasta, proceed to the next steps.
Preheat oven to 375°F.
Cook the mushrooms in 3 Tbsp of the butter over moderate heat, stirring, until all of the liquid they give off has evaporated, about 10 minutes. Set aside.
In a large, heavy saucepan, melt 1/4 cup of butter. Stir in the flour, and cook the mixture over low heat, stirring, for 3 minutes.
Gradually stir in the milk, cream, broth, and the wine or sherry. Bring to a simmer and cook over medium-low heat, stirring constantly, for about 5 minutes.
In a large bowl combine well the pasta, the sauce, the mushrooms, the turkey, the peas, and salt and pepper to taste. Stir in 1/3 cup of the Parmesan and the 1/3 cup of Swiss cheese, and transfer the mixture to a buttered shallow 3-quart casserole.
In a small bowl combine well the remaining 1/3 cup Parmesan, the bread crumbs, and salt and pepper to taste, sprinkle the mixture evenly over the Tetrazzini, and dot the top with the remaining 1 tablespoon butter, cut into bits.
Bake the Tetrazzini in the middle rack of the oven for 30 to 40 minutes, or until it is bubbling and the top is golden.