Heat a large, heavy-bottomed skillet over medium-high heat. Melt the butter in the skillet, then add the chicken cubes and brown on all sides for 2-3 minutes.
Add the garlic and sauté for 30 seconds. Add the pasta, chicken stock, and whole milk, giving it a good stir. Put the lid on the skillet and bring the liquid to a boil, then turn it down and let simmer for 10 minutes (or as long as required for the pasta).
Add the basil pesto, cream cheese, Parmesan, sundried tomatoes, and spinach. Stir it all together and let it simmer for another 10 minutes. Make sure it doesn’t stick to the bottom by keeping the stove on low heat and stirring occasionally. Season to taste with salt and pepper and serve immediately.