Bring 2 quarts of water to a boil with 2 teaspoons of salt. Add the spaghetti and cook following the package directions. Drain well.

Meanwhile, melt the butter in a large skillet or sauté pan over medium heat. Add the onion and mushrooms and cook, frequently stirring, until the onions are translucent and the liquid from the mushrooms has evaporated.

Add the flour to the skillet and cook, stirring, for 2 minutes.

Add the bouillon granules or base and a dash of nutmeg, and gradually stir in the milk; continue cooking, stirring, until thickened. 

Add the turkey, pimientos, and half of the Parmesan cheese and stir to combine. Stir in the hot cooked spaghetti and heat through. 

Transfer the tetrazzini to a serving dish and top with the remaining Parmesan cheese.

Serve with a tossed salad, garlic bread, or a crusty baguette.