Cook the Pasta:

• Bring a large pot of salted water to a boil.
• Cook the spaghetti until just al dente, following the package timing. The pasta should still have a little firmness, especially if you plan to bake the finished dish.
• Drain the spaghetti and set it aside.

Start the Sauce:

• Set a large saucepan or Dutch oven over medium heat.
• Add the butter. Once melted, add the diced onion.
• Cook for 3 to 5 minutes, stirring now and then, until the onion has softened.
• Add the garlic and cook for about 30 seconds, just until fragrant.

Make the Roux:

• Stir in the flour.
• Cook for 2 minutes, stirring often. This helps remove the raw flour taste and gives the sauce a better texture.

Add the Liquids:

• Slowly whisk in the chicken stock.
• Add the milk and keep whisking until the sauce is smooth.
• Stir in the sour cream.
• Cook for 3 to 5 minutes, stirring often, until the sauce has thickened enough to coat the back of a spoon.

Add the Cheese and Seasoning:

• Reduce the heat to low.
• Add the grated white cheddar in handfuls, stirring after each addition until melted before adding more.
• Stir in the diced tomatoes with green chilies, chili powder, cumin, cayenne, salt, and black pepper.
• Taste the sauce and adjust the seasoning.

Add the Chicken and Pasta:

• Add the shredded chicken and cooked spaghetti to the sauce.
• Use tongs to fold everything together until the pasta and chicken are coated.
• Stir in the chopped cilantro just before serving.

Serve from the Pot:

• Serve warm with diced tomatoes and extra cilantro, if using.

Or Bake as a Casserole:

• Heat the oven to 350°F (175°C).
• Transfer the spaghetti mixture to a greased 9-by-13-inch baking dish.
• Cover with foil and bake for 20 to 25 minutes, until hot throughout.
• Remove the foil for the last 5 to 10 minutes if you want the top to set a little.

Make-Ahead Instructions

• The full dish can be assembled one day ahead.
• Cook the pasta just to al dente so it does not become too soft after reheating.
• Transfer the finished spaghetti mixture to a greased baking dish, cover tightly, and refrigerate overnight.
• The next day, let the dish sit at room temperature for about 30 minutes while the oven heats.
• Before baking, check the texture. If the pasta has absorbed too much sauce, stir in ¼ to ½ cup milk or chicken stock.
• Bake covered at 350°F (175°C) for 25 to 30 minutes, or until hot in the center.
• Remove the foil for the last 5 to 10 minutes if desired.