Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package instructions until al dente, about 8-10 minutes. Drain and set aside.

In a medium mixing bowl, beat one egg until frothy. Stir in the cooked chicken, sour cream, cream of chicken soup, Monterey Jack cheese, Parmesan cheese, chopped spinach, half of the French fried onions, garlic, and onion powder. Mix until well combined.

Gently fold the drained spaghetti into the mixture, ensuring each strand is coated. Use a spatula to combine evenly, being careful not to break the pasta.

Preheat your oven to 350ºF (175ºC). Transfer the spaghetti mixture into a greased 9x13-inch baking dish, spread evenly, and smooth the top with a spatula. Cover with aluminum foil and bake for 30 minutes.

Remove the foil, sprinkle the remaining French fried onions on top, and bake uncovered for an additional 5 minutes, or until the onions are golden brown.