Preheat the oven to 400°F (200°C) and lightly grease a 9×13-inch baking dish.

Cook the pasta for 2 minutes less than the package directions, ensuring it is just under al dente. Drain and set aside.

Heat olive oil in a large skillet over medium-high heat. Season the chicken with salt and pepper, and sauté until cooked through, about 5-6 minutes. Remove from heat.

Melt butter in the same skillet over medium heat. Add the minced garlic and sauté for 30 seconds. Stir in the heavy cream and whole milk, bringing it to a gentle simmer for 2-3 minutes. Mix in the basil pesto and Parmesan, stirring until smooth.

Return the chicken, drained pasta, and spinach (if using) to the sauce. Toss until evenly coated.

Transfer to the prepared baking dish and sprinkle mozzarella and extra Parmesan over the top.

Bake uncovered for 20-25 minutes until the cheese is golden and bubbly.

Let rest for 5 minutes before serving.