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

Boil a large pot of salted water and cook the pasta for 2 minutes less than the package instructions. Drain and set aside.

In a large skillet over medium-high heat, heat the olive oil. Season the diced chicken with salt and pepper, then sauté until cooked through and lightly golden, about 5-6 minutes. Remove from heat and set aside.

In the same skillet, melt the butter over medium heat, then add minced garlic. Sauté for about 30 seconds until fragrant.

Slowly stir in the heavy cream and whole milk, bringing it to a gentle simmer to let the sauce thicken slightly. Mix in the basil pesto and grated Parmesan until smooth.

Return the cooked chicken, drained pasta, and optional spinach to the sauce. Toss everything together until evenly coated.

Transfer the mixture into the prepared baking dish. Top with shredded mozzarella and extra grated Parmesan.

Bake uncovered for 20-25 minutes, or until the cheese is golden and bubbly. Let it rest for 5 minutes before serving.