Step 1: Preheat your oven to 375°F (190°C). Cook the spaghetti according to package directions until al dente. Drain well and set aside.
Step 2: While the spaghetti cooks, shred or dice your cooked chicken into bite-sized pieces. Set aside.
Step 3: In a large pot or Dutch oven, melt 4 tablespoons of butter over medium heat. Add the diced onion and cook until softened and translucent, about 5-7 minutes.
Step 4: Stir in the minced garlic and cook for another minute until fragrant. Be careful not to let the garlic brown.
Step 5: Sprinkle the flour over the onion and garlic mixture. Cook, stirring constantly, for 1-2 minutes to create a roux. This step helps cook out the raw flour taste.
Step 6: Gradually whisk in the chicken broth and milk, a little at a time, ensuring there are no lumps. Bring the mixture to a gentle simmer, stirring occasionally, until the sauce thickens.
Step 7: Reduce the heat to low. Stir in the softened cream cheese until fully melted and smooth. Then, slowly add the shredded cheddar cheese and Monterey Jack cheese, stirring until they are completely melted and the sauce is creamy. Season with salt, black pepper, and paprika to taste.
Step 8: Add the cooked chicken and drained spaghetti to the creamy cheese sauce. Toss gently until everything is evenly coated.
Step 9: Pour the creamy chicken spaghetti mixture into a 9x13 inch baking dish.
Step 10: In a small bowl, combine the breadcrumbs and 2 tablespoons of melted butter. Sprinkle this mixture evenly over the top of the spaghetti in the baking dish.
Step 11: Bake in the preheated oven for 20-25 minutes, or until bubbly and the breadcrumb topping is golden brown.
Step 12: Remove from the oven and let it rest for 5-10 minutes before serving. Garnish with fresh chopped parsley, if desired. Serve hot and enjoy your delicious creamy chicken spaghetti recipe!