Heat the oven and start the pasta. Heat the oven to 375 F (190 C). Lightly grease a 9x13-inch baking dish. Bring a large pot of salted water to a boil and cook the spaghetti until just shy of al dente, about 1 minute less than the package time. Drain well and set aside.

Cook the mushrooms and onion. While the pasta cooks, melt the butter in a large deep skillet over medium-high heat. Add the mushrooms and onion and cook for 5 to 6 minutes, until the mushrooms release their liquid and start to brown and the onion is soft. Stir in the garlic and cook for 1 minute.

Make the sauce. Sprinkle the flour over the mushrooms and stir for 1 minute. Pour in the broth a little at a time, whisking constantly so no lumps form. Stir in the half-and-half and the sherry, if using. Simmer gently for 3 to 4 minutes, until the sauce coats the back of a spoon. Season with Italian seasoning, nutmeg, salt and pepper.

Add the turkey and pasta. Take the skillet off the heat. Stir in the diced turkey, the peas, half of the mozzarella and half of the Parmesan. Add the drained spaghetti and toss until every strand is coated.

Top and bake. Spread the mixture in the prepared baking dish. In a small bowl, toss the panko with the olive oil and the remaining Parmesan, then scatter it over the top with the remaining mozzarella. Bake uncovered for 25 minutes, until bubbly at the edges and golden on top.

Rest and serve. Let the tetrazzini rest for 10 minutes before serving so the sauce settles. Sprinkle with fresh parsley and serve hot.