Turkey Tetrazzini

Name: Turkey Tetrazzini
Ingredients: 6 tbsp. unsalted butter; 1 medium yellow onion, chopped; 1 lb. button mushrooms, thinly sliced; 1 red bell pepper, seeded, stemmed, and chopped; 1⁄4 cup flour; 1 cup chicken or turkey broth; 1 cup milk; 2 cups diced cooked turkey; 1 1⁄2 cups shredded cheddar cheese, divided; 1⁄2 cup grated parmesan cheese; 2 tbsp. sherry; 1 lb. spaghetti; 1⁄2 cup frozen peas; Kosher salt and freshly ground black pepper