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

In a large pot of boiling salted water, cook pasta for 2 minutes less than package directions to ensure it's just under al dente. Drain and set aside.

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

In the same skillet, melt butter over medium heat. Add minced garlic and sauté for 30 seconds until fragrant.

Stir in heavy cream and whole milk, then bring to a gentle simmer for 2–3 minutes until slightly thickened.

Mix in basil pesto and grated Parmesan, stirring until smooth and melted.

Return cooked chicken, drained pasta, and spinach (if using) to the sauce. Toss thoroughly until all ingredients are evenly coated.

Transfer the coated mixture into the prepared baking dish. Evenly sprinkle shredded mozzarella and extra Parmesan cheese over the top.

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

Allow dish to rest for 5 minutes before serving.