12 ounces mushrooms, sliced (about 4-5 cups)
1/2 cup unsalted butter
1/4 cup all-purpose flour
1 1/2 cups of milk
1/4 cup cream
2 cups chicken broth
1/4 cup dry sherry (or vermouth/dry white wine)
3 cups coarsely chopped cooked turkey
1 cup peas
2/3 cup freshly grated Parmesan (divided into 1/3 and 1/3 cups)
1/3 cup shredded Swiss cheese
2 Tbsp lemon juice
Salt and Pepper
Ground nutmeg (optional)
1/3 cup fresh bread crumbs (I use Panko)
Freshly chopped parsley for garnish (optional)
Into the saucepan with the butter and flour, 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 bowl 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 (I used cheddar because that was all I had and it was still delicious!). 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 is seasoned to your taste. Add a pinch of ground nutmeg if using, again to taste. Transfer the mixture to a greased 3-quart casserole.
In a small bowl combine the remaining 1/3 cup Parmesan and the bread crumbs. 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.
Garnish individual servings with chopped parsley, if desired.