Preheat the oven to 400°F (200°C). Lightly grease a 9x13-inch baking dish.

Bring a large pot of salted water to a boil. Cook the pasta for 2 minutes less than the package directions. Drain and set aside.

Heat the olive oil in a large skillet over medium-high heat. Season the diced chicken with salt and pepper. Sauté until cooked through and lightly golden, about 5 to 6 minutes. Transfer the chicken to a bowl and set aside.

Reduce the skillet heat to medium and add the butter. When it melts, stir in the minced garlic and cook 30 seconds until fragrant.

Pour in the heavy cream and whole milk. Stir and bring to a gentle simmer for 2 to 3 minutes so the mixture slightly thickens.

Stir the basil pesto and 3/4 cup grated Parmesan into the cream until smooth and fully combined. Taste and adjust salt and pepper.

Return the cooked chicken to the skillet. Add the drained pasta and spinach if using. Toss until the pasta and chicken are evenly coated in the sauce.

Transfer the mixture into the prepared baking dish. Sprinkle the shredded mozzarella and the extra 1/4 cup Parmesan evenly on top.

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

Remove from the oven and let the dish rest for 5 minutes before serving.