Preheat the oven to 375°F and lightly grease a 9×13-inch baking dish.

In a skillet, heat oil and cook the turkey sausage until fully browned. Add onions, bell peppers, mushrooms, and garlic; sauté until veggies are tender.

Stir in chopped spinach, Italian seasoning, smoked paprika, red pepper flakes (optional), salt, and pepper until evenly mixed.

In a large bowl, whisk together eggs, cottage cheese, and milk until well combined.

Gently fold the cooked turkey sausage and vegetable mixture into the egg mixture.

Pour the mixture into the prepared baking dish and spread evenly.

Sprinkle the top with shredded cheddar cheese.

Bake uncovered for 40–50 minutes, or until the center is set and the edges are golden brown.

Let the casserole cool slightly before slicing into squares and serving.