Preheat oven to 350*F (175*C). Lightly grease a 10 x 6 x 2-inch baking dish. Set aside.
In a large bowl, combine egg, onion, sour cream, milk, salt, pepper, and 2 tablespoons Parmesan cheese. Add Monterey Jack cheese; mix well.
Add the spinach and cooked spaghetti and toss to combine well. Place mixture in the prepared baking dish and sprinkle with remaining Parmesan.
Bake for 15 minutes covered; remove cover and continue baking for additional 15 minutes.