01 - Preheat the oven to 400°F. Lightly grease a 9x13-inch baking dish and set aside.
02 - Bring a large pot of salted water to a rolling boil. Add the pasta and cook 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 cook until golden brown and cooked through, approximately 6 to 7 minutes. Remove from heat and set aside.
04 - In a large mixing bowl, toss together the drained pasta, seared chicken, basil pesto, heavy cream, milk, and half of the shredded mozzarella. If using cherry tomatoes or baby spinach, fold them in at this stage. Mix until everything is evenly coated.
05 - Transfer the pasta mixture into the prepared baking dish, spreading it into an even layer. Scatter the remaining mozzarella and all of the grated Parmesan evenly over the top.
06 - Place the dish in the preheated oven and bake for 20 minutes, or until the cheese is melted, bubbly, and lightly golden on top.
07 - Remove from the oven and let the bake stand for 5 minutes to allow the sauce to set. Garnish with fresh basil leaves if desired, then serve warm.