Bring a large pot of salted water to a boil. Add the penne pasta and cook according to package directions until al dente. During the last 3 minutes of cooking, add the broccoli florets directly to the pasta water.

Before draining, reserve 1 cup of starchy pasta water in a measuring cup. Drain the pasta and broccoli together and set aside.

In your largest skillet, heat olive oil and butter over medium-low heat. Add the diced onion and cook for 4-5 minutes until softened.

Stir in the minced garlic and cook for another 30 seconds until fragrant.

Pour in the heavy cream and chicken broth, bringing the mixture to a gentle simmer for 2-3 minutes while avoiding a rolling boil.

Remove the skillet from heat completely before adding cheese to prevent graininess. Whisk in the Parmesan and mozzarella until fully melted and smooth.

Season the sauce with Italian seasoning, garlic powder, salt, pepper, and red pepper flakes.

Gently fold the drained pasta and broccoli into the creamy sauce using tongs, ensuring everything is coated evenly.

Lastly, add the shredded rotisserie chicken.

Incorporate the reserved pasta water 2 tablespoons at a time until you achieve a creamy, glossy consistency. For an extra touch of richness, stir in a cold knob of butter just before serving.