Chicken Sausage and Potato Casserole

Name: Chicken Sausage and Potato Casserole
Ingredients: 3 (4-ounce) chicken italian sausage links, casings removed (turkey may be substituted); 1 tablespoon butter; 3 cups chopped onion; ¾ cup chopped celery hearts; 8 ounces baby bella mushrooms, chopped; 1½ pounds red potatoes, cubed into bite-sized pieces (about ½-in cubes); ½ teaspoon kosher salt; ¼ teaspoon fennel seed; ¾ cup fat-free, lower-sodium chicken broth; Pinch of red pepper flake; Cooking spray; ¾ cup (3 ounces) shredded swiss cheese; 2 tablespoons chopped fresh thyme