Preheat your oven to 400 degrees. This is important because you want your casserole to bake evenly. The heat will help it bubble beautifully as it cooks.
In a large pot, bring 4 quarts of water to a boil, then add 1 tablespoon of salt. This ensures your spaghetti is well-seasoned. Once the water is boiling, add the spaghetti and cook until al dente, about 9 to 10 minutes. You want it cooked but still slightly firm. Drain well once done.
While the spaghetti is cooking, heat 1 tablespoon of olive oil in a large skillet over medium-high heat until it shimmers. This means it's hot enough to add your vegetables. Add the chopped onion and red bell pepper and cook until softened, about 5 minutes. You’ll notice the vibrant colors and sweet aromas—this is a good sign!
Add the chicken pieces and 1 teaspoon of chili powder to the skillet. Season with salt and pepper to taste; I suggest 1 teaspoon of salt and a ½ teaspoon of pepper. Stir everything together, allowing the chicken to cook until it turns golden brown and is no longer pink in the center, about 5 to 7 minutes. Once cooked, transfer this mixture to a bowl and set aside.
Now, return the same skillet to medium-high heat and melt 1/4 cup of butter. Be careful not to let it burn. After it melts, whisk in 1/4 cup of flour, cooking for about 1 minute until lightly browned. This creates a roux, which is essential for your creamy sauce.
Slowly whisk in 2 cups of milk and 1 cup of chicken broth, stirring continuously until the mixture thickens, about 3 to 5 minutes. You want it to coat the back of a spoon. Don’t skip this step; it adds so much creaminess to your Chicken Spaghetti.
Season the sauce with salt and pepper again, tasting for balance. Stir in 1 cup of shredded cheddar cheese until it melts completely.
Next, add the cooked spaghetti and the chicken and vegetable mixture back into the skillet. Toss everything together until well combined. You should see the pasta coated in that beautiful creamy sauce.
Transfer the entire mixture to a 3-quart or larger baking dish. Sprinkle the remaining 1 cup of shredded cheddar cheese on top. This will create a delicious cheesy crust.
Bake in your preheated oven for about 20 to 25 minutes, or until hot and bubbly. Make sure the chicken registers 165 degrees on an internal thermometer, ensuring it’s safely cooked.
Once done, let it stand for 5 minutes before serving. This makes it easier to scoop out and enjoy. Garnish with freshly minced parsley if desired, and get ready for the compliments!
