Preheat a large pot of salted water to boiling. Add the spaghetti pasta and cook according to package instructions until al dente (about 8-9 minutes). Drain the pasta and set aside.

If chicken is not cooked, poach or bake chicken breasts until cooked through (about 15 minutes poaching or 25 minutes baking at 375°F). Let cool slightly, then shred or chop into bite-sized pieces.

In a medium skillet, melt the butter over medium heat. Once melted, whisk in flour and cook for about 1-2 minutes until it forms a paste and begins to bubble but doesn’t brown.

Slowly whisk in the milk and chicken broth, continuing to whisk to avoid lumps. Cook, stirring frequently, until the sauce thickens and coats the back of a spoon, about 5-7 minutes.

Add garlic powder, onion powder, black pepper, and salt to the sauce. Stir well and remove from heat.

Preheat your air fryer to 360°F (180°C) or oven to 350°F (175°C).

In a large mixing bowl, combine cooked spaghetti, shredded chicken, diced bell peppers, chopped onion, half of the shredded cheddar cheese, half of the mozzarella cheese, and the prepared sauce. Mix until everything is evenly coated.

Transfer the mixture into a greased 9x13 inch casserole dish, spreading it out evenly.

Sprinkle the remaining cheddar and mozzarella cheese evenly on top.

Place the casserole dish in the air fryer basket (if the casserole dish fits) or bake in the oven. Cook for 20-25 minutes until the cheese is melted, bubbly, and lightly golden on top.

If using the air fryer, check at 15 minutes to ensure it doesn’t burn; cover loosely with foil if the topping browns too quickly.

Remove the casserole from heat and let rest for 5-10 minutes before serving to allow the casserole to set slightly.