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

In a skillet, heat olive oil over medium heat. Add chopped onion, bell pepper, and sliced mushrooms. Sauté for 5-7 minutes until soft and caramelized.

Reduce heat to low. Stir in cream of chicken soup, sour cream, garlic powder, onion powder, and milk until smooth. Cook until heated through, but do not boil.

Add shredded cooked chicken and half of the Monterey Jack cheese to the sauce, mixing until the cheese is melted.

Carefully add drained pasta to the skillet and toss until well-coated in the creamy sauce.

For optional baking, preheat oven to 350°F (175°C). Transfer mixture to a greased baking dish, top with remaining cheeses, and bake for 20-25 minutes.

Remove from the oven, garnish with chopped parsley, and serve warm.