Cheesy Spinach and Mushroom Breakfast Stuffing

Name: Cheesy Spinach and Mushroom Breakfast Stuffing
Ingredients: 1 (16- to 18-ounce) loaf unsliced French or Italian bread; Cooking spray or butter, for the baking dish; 1 pound cremini mushrooms; 1 medium yellow onion; 1 teaspoon dried thyme; 1 1/4 teaspoons kosher salt, divided, plus more as needed; 1/4 teaspoon freshly ground black pepper, plus more as needed; 4 tablespoons (1/2 stick) unsalted butter; 8 ounces Swiss cheese, shredded (about 2 cups); 1 (5 to 6-ounce) bag baby spinach (5 to 6 packed cups); 1 1/2 cups low-sodium vegetable or chicken broth