01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or cooking spray.
02 - Bring a large pot of salted water to a boil. Cook spaghetti according to package directions until al dente. Drain well and set aside.
03 - In a large skillet over medium heat, melt the butter. Add diced onions and bell peppers; sauté for 3–4 minutes until softened. Add minced garlic and cook 1 minute longer until fragrant.
04 - Stir flour into the skillet and cook for 1 minute, stirring constantly. Gradually whisk in milk and chicken broth. Continue cooking and whisking for 3–4 minutes until the sauce thickens and bubbles.
05 - Add Monterey Jack cheese, sharp cheddar cheese, salt, black pepper, and smoked paprika to the sauce. Stir continuously until all cheese is melted and the sauce becomes smooth and creamy.
06 - Remove the skillet from heat. Stir in sour cream until fully incorporated, then add the cooked chicken and drained spaghetti. Toss everything together until evenly coated with sauce.
07 - Transfer the mixture to the prepared baking dish, spreading it evenly. Sprinkle the remaining 1/2 cup of Monterey Jack cheese over the top in an even layer.
08 - Bake for 20–25 minutes until the cheese is melted, bubbly, and lightly golden on top. The edges should be slightly crispy.
09 - Remove from oven and let rest for 5 minutes. Garnish with thinly sliced green onions before serving hot.