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

Bring a large pot of salted water to a boil. Cook the pasta for 2 minutes less than the package directions so it remains slightly under al dente. 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 the chicken in batches if needed until cooked through and lightly golden, about 5–6 minutes. Transfer the chicken to a plate and remove from heat.

In the same skillet, reduce heat to medium and melt the butter. Add the minced garlic and cook for about 30 seconds until fragrant—don’t let it brown.

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

Stir the basil pesto and 3/4 cup grated Parmesan into the cream sauce. Keep stirring until the sauce is smooth and the cheese is melted. Taste and season with more salt and pepper if needed.

Return the cooked chicken and drained pasta to the skillet. Add the baby spinach now if using. Toss everything until evenly coated.

Transfer the coated mixture to the prepared baking dish. Evenly sprinkle the shredded mozzarella and the remaining 1/4 cup grated Parmesan over the top.

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

Let the dish rest for 5 minutes before serving so the sauce sets and slices hold together.