Preheat your oven to 400 degrees F and lightly grease a 10 inch baking dish.
Cook the pasta in salted water until just al dente. Drain and set aside.
In a large pan, melt the butter over medium heat. Add the chopped onion and cook until soft.
Stir in the garlic and cook for about 30 seconds until fragrant.
Add cream cheese, milk, and sour cream. Stir often and cook gently until sauce is smooth.
Stir in the Parmesan cheese, Italian seasoning, salt, and pepper.
Add the chicken, spinach, cooked pasta, chopped tomatoes, and 1 cup mozzarella cheese.
Transfer mixture to the prepared baking dish. Top with remaining mozzarella cheese.
Bake uncovered for 20 to 25 minutes, until bubbly and lightly golden.
Let rest for 5 minutes before serving.
