Preheat oven to 350°. Spray a 2-quart casserole dish with non-stick spray, and set aside.
Cook sausage and onion in a frying pan, just until the sausage is browned and the onion is softened. Drain off any excess grease if necessary.
Add the cream cheese, stir until melted, and combined with the sausage. Remove from heat.
In a small mixing bowl, mix the eggs, sour cream, and milk together. Stir in ½ cup of the shredded cheese and the parmesan cheese.
Pour the cream mixture into the sausage mixture, add the spinach and mix well.
Pour the mixture into the greased casserole dish. Top with the remaining ½ cup of cheese.
Bake for 30 minutes, until the cheese is melted and bubbling.