Preheat oven 350 F. Prepare a square 8x8 baking dish by spraying with cooking spray.Bring a medium skillet to medium heat and add the turkey sausage. With a wooden spoon, break down the sausage as it browns. Once browned, drain the meat then set aside.In a large mixing bowl, whisk together egg whites with milk, salt, pepper and seasonings until frothy.Assemble the casserole by evenly layering the spiralized sweet potato on the bottom of the casserole dish.Then add the meat over the top in an even layer.Layer the spiralized zucchini and spinach on top.Pour the egg white mixture over the top of the casserole.Bake in the oven at 350 F 30-37 minutes uncovered.Remove from the oven and cool 10 minutes in the baking dish before slicing into 9 large portions.