Preheat oven to 375 degrees. Spray a 4 quart baking dish with cooking spray.
Bring a large pot of heavily salted water to a boil. Add the spaghetti and cook according to the package directions. Drain, rinse with hot water, and drain again. Reserve.
Heat 3 tablespoons oil in a large skillet over medium-high heat, add the onions and cook until soft, about 3 minutes. Add the garlic and cook for 30 seconds. Add the mushrooms, season with a big pinch of salt and pepper, cook until the mushroom release liquid and then all the liquid is evaporated, about 8-10 minutes. Add the sherry and cook until the sherry is evaporated. Add the flour, cook for 1 minute then add the chicken stock and half-and-half, and cook until the mixture starts to thicken, about 10 minutes, stirring constantly. Add the turkey, ¾ cup Parmesan cheese, the peas and reserved spaghetti. Stir to combine. Taste and add more salt and pepper if desired. Pour the mixture into the prepared baking dish.
Combine the bread crumbs with the remaining 2 tablespoons of olive oil, remaining 1/4 cup Parmesan cheese, and the minced parsley. Sprinkle over the top of the casserole and bake for 30 minutes or until the browned on top and bubbly.