Cook the spaghetti according to package instructions until al dente.
In a large bowl, combine shredded chicken, barbecue sauce, sour cream, garlic powder, and black pepper.
Once the spaghetti is cooked, drain it and add it to the bowl with the chicken mixture.
Stir in half of the shredded Monterey Jack cheese and half of the chopped green onions.
Transfer the mixture to a greased baking dish, spreading it evenly.
Top with the remaining Monterey Jack cheese and green onions.
Bake in a preheated oven at 350°F (175°C) for 20-25 minutes or until bubbly and golden on top.
Remove from the oven and let it cool for a few minutes before serving.