01 - Preheat the oven to 400°F. Lightly grease a 9x13-inch baking dish with olive oil or cooking spray.
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 well 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, 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. Fold in cherry tomatoes and baby spinach if using. Stir until everything is evenly coated.
05 - Transfer the pasta mixture into the prepared baking dish, spreading it into an even layer. Sprinkle the remaining mozzarella and all of the grated Parmesan evenly over the top.
06 - Bake uncovered for 20 minutes, or until the cheese is melted, bubbly, and lightly golden on top.
07 - Remove from the oven and let stand for 5 minutes before serving. Garnish with fresh basil leaves if desired.