01 - Preheat the oven to 400°F. Lightly grease a 9x13 inch baking dish.
02 - Bring a large pot of salted water to a rolling boil. Cook the pasta according to package directions until al dente. Drain thoroughly and set aside.
03 - While the pasta cooks, heat olive oil in a large skillet over medium-high heat. Add the diced chicken, season generously with salt and pepper, and sauté until golden brown and cooked through, about 6 to 7 minutes. Remove from heat.
04 - In a large mixing bowl, toss the cooked pasta and chicken with the basil pesto, heavy cream, milk, and half of the mozzarella cheese. Fold in the cherry tomatoes and baby spinach if desired, ensuring the sauce thoroughly coats all ingredients.
05 - Transfer the coated pasta mixture into the prepared baking dish. Sprinkle the remaining mozzarella and the Parmesan cheese evenly over the top.
06 - Bake uncovered in the preheated oven for 20 minutes, or until the sauce is bubbling and the cheese has melted into a golden crust.
07 - Remove from the oven and allow the dish to rest for 5 minutes to set before slicing. Garnish with fresh basil leaves if desired.