Preheat your oven to 350°F (175°C).
In a large skillet, cook the turkey breakfast sausage over medium heat until browned and fully cooked through, about 5-7 minutes. Drain any excess fat if necessary.
Add the fresh baby spinach to the skillet and cook until wilted, about 2-3 minutes. Remove from heat and set aside to cool slightly.
In a large mixing bowl, whisk together the eggs, milk, kosher salt, and black pepper until well combined and frothy.
Add the cooked sausage, wilted spinach, sun-dried tomatoes, mozzarella cheese, and parmesan cheese to the egg mixture. Gently fold until evenly distributed.
Pour the mixture into a greased 9x13-inch baking dish and spread it out evenly.
Bake for 35-40 minutes, or until set in the center and golden on top. Check for doneness by inserting a knife in the center; it should come out clean.
Allow the casserole to cool for about 10 minutes before slicing and serving.
