Bring a large pot of salted water to a rolling boil over high heat. Add the penne pasta and cook for 8–10 minutes until al dente, adding broccoli florets for the final 3 minutes.

Carefully drain the pasta and broccoli together, reserving 1 cup of starchy pasta water. Set aside while preparing the sauce.

In a large skillet, heat 2 tablespoons of olive oil and 2 tablespoons of butter over medium-low heat until melted. Add diced yellow onion and sauté for 3–4 minutes until softened. Incorporate minced garlic and cook for 1 additional minute.

Pour in 1 cup of heavy cream and 1 cup of low-sodium chicken broth, stirring to combine. Simmer over medium heat for 2–3 minutes without boiling.

Remove the skillet from heat and whisk in 1 cup of grated Parmesan and 1 cup of shredded mozzarella until melted and smooth.

Stir in 1 teaspoon of Italian seasoning, ½ teaspoon garlic powder, and a pinch of salt and pepper. Add ¼ teaspoon of red pepper flakes if desired.

Gently add the drained penne and broccoli to the sauce, then fold in the shredded rotisserie chicken, adjusting sauce consistency with reserved pasta water.

Stir in a cold knob of butter just before serving for added richness. Serve warm.