Turkey Spaghetti

Name: Turkey Spaghetti
Ingredients: Kosher salt; 12 ounces spaghetti; 4 tablespoons (1/2 stick) salted butter; 1/2 cup diced celery; 1/2 cup diced carrot; 1 tablespoon chopped fresh sage; 2 cloves garlic, minced; 1 small onion, diced; 1/4 teaspoon freshly ground black pepper; 2 cups diced roasted turkey (or substitute rotisserie chicken); One 10.5-ounce can cream of mushroom soup; 1 cup leftover gravy (or substitute another can of cream of mushroom soup); 1/2 cup stock or water; 1 1/2 cups grated Cheddar cheese; Chopped fresh parsley, for serving