Step 1

Preheat the oven to 350 degrees F. Coat a 9"x13" pan with cooking spray.

Step 2

Cook the spaghetti in a large pot of salted water according to package directions.

Step 3

Melt the butter in a large skillet over medium heat. Add the onion and cook for 3-4 minutes or until softened.

Step 4

Add the garlic and cook for 30 seconds. Stir in the flour and cook for 1 minute.

Step 5

Slowly add the milk to the pan, whisking constantly.

Step 6

Bring the milk mixture to a simmer and cook for 3-4 minutes or until just thickened.

Step 7

Remove the sauce from the heat, and mix in 2 1/2 cups of cheddar cheese, along with the cream cheese. Stir until cheeses are melted. Season with salt and pepper to taste.

Step 8

Drain the spaghetti and return it to the cooking pot. Add the chicken and tomatoes to the pasta.

Step 9

Pour the cheese sauce over the pasta and toss to coat evenly.

Step 10

Place the pasta mixture into the prepared pan. Sprinkle the remaining 1 1/2 cups of cheddar cheese over the top.

Step 11

Bake for 20-25 minutes, until casserole is bubbly and cheese is melted. Sprinkle with parsley, then serve.