In the microwave, scald the milk and turkey stock almost to boiling.
In the saucepan or Dutch oven that you cooked the pancetta and mushrooms, melt the butter and add the flour, black pepper, salt, nutmeg and garlic.
Cook together over medium heat for 2 minutes until foamy.
Whisking constantly, slowly pour in scalded milk and stock mixture in about three additions, allowing the sauce to thicken and come back to a simmer after each addition.
Continue to cook for 2 more minutes stirring constantly.
Grease an 8 - 10 cup casserole dish.
In a large bowl, stir together the pasta, turkey, pancetta, mushrooms, bell pepper and sauce.
When well combined stir in the 1/2 cup finely grated Parmesan cheese.
Pour into the prepared casserole dish.