Recipe: Slow Cooker Sausage and Spinach Breakfast Casserole

Name: Recipe: Slow Cooker Sausage and Spinach Breakfast Casserole
Ingredients: Cooking spray; 1 pound uncooked pork breakfast sausage, casings removed; 1 small yellow onion, diced; 1 1/2 teaspoons dried oregano; 5 ounces baby spinach (about 5 packed cups); 1 pound frozen diced potatoes (do not thaw, about 4 cups); 1 1/4 cups shredded Swiss cheese, divided; 1/4 cup grated Parmesan cheese, plus more for serving; 8 large eggs; 2 cups whole or 2% milk; 2 teaspoons Dijon mustard; 1 1/2 teaspoons kosher salt; 1/4 teaspoon freshly ground black pepper; Red pepper flakes or hot sauce, for serving