cup fine soft bread crumbs (if using bread crumbs make sure they are fresh and chopped to a fine texture) or 1/3 cup panko breadcrumbs (if using bread crumbs make sure they are fresh and chopped to a fine texture)
Preheat oven to 375 degrees F. Start heating 2 to 3 quarts of water for the pasta. Add 1 tsp of salt for each quart of water.
Cook the mushrooms in 3 Tbsp of the butter over medium heat, stirring until all of the liquid the mushrooms give off has evaporated, 5-10 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.
About now, put the pasta into the boiling water you've heated. Follow the package directions and cook until al dente. While the pasta is cooking continue on with the recipe.
In a saucepan add the butter and flour, and slowly whisk in the milk, cream, broth, and the sherry. Bring to a simmer and cook over medium-low heat, stirring constantly, for about 5 to 8 minutes.
When the pasta is ready, drain it. In a large bowel combine the pasta, the sauce, the mushrooms, the turkey, and the peas. Stir in 1/3 cup of the Parmesan and the 1/3 cup of Swiss cheese. Stir in the lemon juice. Add salt and pepper to taste. Note that if you have been using unsalted butter, and/or unsalted or low sodium stock, you will need to add more salt than you might expect. Just keep sprinkling it in until it i seasoned to your taste. Add a pinch of ground nutmeg if using, again to taste. Transfer the mixture to a buttered 3-quart casserole (or if you increase the serving size, use the appropriate size dish).
In a small bowl combine well the remaining 1/3 cup Parmesan and the bread crumps. Sprinkle the mixture evenly over the tetrazzini and dot the top with the remaining 1 Tbsp butter, cut into bits. Use more butter, if needed to cover the casserole with butter with about a bits every 3 inches.
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. If top is browning too quickly, you can cover with foil.