Cook spaghetti according to package directions. Meanwhile, in a large bowl, beat egg. 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-qt. baking dish. Cover and bake at 350° for 30 minutes or until heated though. Top with remaining onions; return to the oven for 5 minutes or until onions are golden brown.