Preheat the oven to 200°C (400°F). Tear the mozzarella and leave it on a piece of kitchen paper to drain while you make the sauce.

Bring a large pot of water to a boil and salt it generously. Cook the rigatoni for 2 minutes less than the package instructions, then drain and set aside.

Heat the olive oil and butter in a large, deep pan over medium heat. Add the chicken, season with salt and pepper, and cook for 5-6 minutes until golden and cooked through. Remove and set aside.

Add the onion and mushrooms to the same pan and cook for 6-7 minutes over medium heat, until the mushrooms start to color.

Stir in the garlic and cook for 1 minute, then add the tomato paste and cook for 1 minute more, until it darkens slightly.

Pour in the chicken stock and tomato sauce, add the torn basil, and simmer for 5 minutes until slightly thickened.

Lower the heat, stir in the half and half and a third of the Parmesan, then add the spinach and cook for 1-2 minutes until wilted. Return the cooked chicken to the pan and season to taste.

Add the drained rigatoni to the pan and toss until everything is well coated.

Transfer to a baking dish. Scatter the torn mozzarella and remaining Parmesan evenly over the top.

Bake uncovered for 20-25 minutes, until bubbling and golden at the edges. Let it rest for 5 minutes before serving.