Cook spaghetti. Meanwhile, beat egg in medium bowl. Add sour cream, Parmesan cheese and garlic powder. Drain spaghetti' add to egg mixture with Monterey Jack cheese, spinach and half of the onions. Pour into a greased 2-quart baking dish. Cover and bake at 350 for 30 minutes or until heated through. Top with remaining onions; return to the oven uncovered for 5 minutes or until onions are golden brown.