Set oven temperature to 350°F. Grease a 9-by-13-inch casserole dish and keep it aside.

In a medium pan set over medium-high heat, add the sausage. Cook until the sausage is barely cooked through, breaking it up with a wooden spoon. Using a slotted spoon, transfer the sausage to a small bowl.

After lowering the heat to medium, add the spinach to the skillet along with the sausage's liquid and oil. Cook until wilted, 2 to 3 minutes. With a slotted spoon, carefully remove the excess liquid from the spinach by pressing it down over the sink with the wooden spoon. As an alternative, you can put the cooked spinach in a fine-mesh sieve and drain it, gently squeezing out any extra liquid. Transfer the spinach to a chopping board and coarsely chop.

In the meantime, place the eggs in a large measuring cup or medium basin. Stir until well combined. Stir in the milk, 1 cup mozzarella, 2 tsp parmesan, pepper, and salt. Mix by whisking.

Arrange the sausage, cooked spinach and sun-dried tomatoes in layers in the prepared casserole dish. Cover with the remaining 1 cup mozzarella and 2 tablespoons parmesan cheese after pouring the egg mixture over the top.

Place in the oven and bake for 35 minutes, rotating once during the baking process if preferred, to provide uniform browning. Serve after slicing into ten pieces.