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 boil. Cook spaghetti until al dente according to package directions. Drain well and set aside.
03 - In a large skillet over medium heat, melt 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 - Sprinkle flour over vegetables and stir constantly for 1 minute to cook out raw flour taste. Gradually whisk in milk and chicken broth, continuing to whisk until smooth. Cook for 3-4 minutes until sauce thickens and coats the back of a spoon.
05 - Remove skillet from heat. Stir in Monterey Jack cheese, cheddar cheese, salt, black pepper, and smoked paprika until completely melted and sauce is smooth and creamy.
06 - Stir sour cream into cheese sauce until well incorporated. Add cooked chicken and drained spaghetti, tossing gently until evenly coated with sauce.
07 - Transfer mixture to prepared baking dish, spreading evenly. Sprinkle remaining Monterey Jack cheese over the top in an even layer.
08 - Bake for 20-25 minutes until cheese is bubbly and lightly golden brown on top.
09 - Remove from oven and let rest for 5 minutes. Garnish with thinly sliced green onions before serving hot.