1. Cook the Spaghetti:

1. Bring a large pot of salted water to a boil. Cook the spaghetti according to package instructions until al dente. Drain and set aside.

2. Make the Sauce:

1. In a large skillet, heat the olive oil or butter over medium heat.
2. Add the diced onion and cook until softened, about 3-4 minutes.
3. Stir in the garlic and cook for 1 minute, until fragrant.
4. Sprinkle the flour over the onion and garlic mixture, stirring to coat. Cook for 1-2 minutes.
5. Gradually whisk in the milk, ensuring no lumps form. Bring the mixture to a simmer and cook until thickened, about 5 minutes.
6. Reduce the heat to low and stir in 1 1/2 cups of Monterey Jack cheese and 1/4 cup of Parmesan cheese until melted and smooth.
7. Add the smoked paprika, cayenne pepper (if using), salt, and pepper.

3. Combine the Ingredients:

1. In a large mixing bowl, combine the cooked spaghetti, shredded chicken, crumbled bacon, and cheese sauce. Mix until everything is well coated.

4. Assemble the Casserole:

1. Preheat your oven to 375°F (190°C).
2. Transfer the mixture to a greased 9×13-inch baking dish.
3. Sprinkle the remaining Monterey Jack cheese and Parmesan cheese evenly over the top.

5. Bake:

1. Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
2. Broil for 1-2 minutes at the end for a golden, crispy top (optional).

6. Serve:

1. Let the casserole cool for a few minutes. Garnish with chopped parsley and serve hot.