Preheat oven to 400 degrees F. Lightly grease a 9x13-inch baking dish and set aside.

Bring a large pot of salted water to a boil. Cook the pasta for 2 minutes less than package directions, about 8 minutes for most brands, so it stays just under al dente. Drain and set aside.

Heat olive oil in a large skillet over medium-high heat. Season the diced chicken with salt and pepper and cook in a single layer for 5 to 6 minutes, stirring occasionally, until lightly golden and cooked through. Remove from the skillet and set aside.

In the same skillet over medium heat, melt the butter. Add the minced garlic and stir for about 1 minute until fragrant and just golden. Do not let it brown.

Pour in the heavy cream and whole milk, scraping up any browned bits from the pan. Bring to a gentle simmer and cook for 2 to 3 minutes until slightly thickened.

Stir in the basil pesto and 3/4 cup Parmesan until smooth and fully melted. Taste and adjust seasoning with salt and pepper as needed.

Add the cooked pasta, sauteed chicken, and baby spinach to the skillet. Toss until evenly coated and the spinach just begins to wilt, about 1 minute.

Transfer the mixture to the prepared baking dish and spread evenly. Sprinkle the mozzarella and remaining 1/4 cup Parmesan over the top.

Bake uncovered for 20 to 22 minutes until the cheese is golden and bubbling. Let rest for 5 minutes before scooping and serving.