Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Cook the penne pasta in a large pot of salted water according to package directions until al dente. Drain well and set aside.

While the pasta is cooking, heat the olive oil in a large skillet or Dutch oven over medium-high heat. Season the chicken cubes with salt and pepper. Add the chicken to the skillet and cook until browned on all sides and cooked through, about 5-7 minutes. Remove the chicken from the skillet and set it aside.

To the same skillet, add the chopped onion and cook until softened, about 3-4 minutes. Add the minced garlic, dried oregano, dried basil, and red pepper flakes (if using). Cook for 1 minute more until fragrant.

Pour in the crushed tomatoes and chicken broth. Bring to a simmer, then reduce the heat to low. Stir in the heavy cream, 1 tsp salt, and 1/2 tsp black pepper. Let the sauce simmer gently for 5 minutes to allow the flavors to meld.

Turn off the heat. Stir the fresh spinach into the hot sauce until it wilts completely. Return the cooked chicken to the skillet along with the drained penne pasta. Stir everything together until the chicken and pasta are well coated in the sauce.

Transfer the chicken and pasta mixture to the prepared 9x13 inch baking dish, spreading it into an even layer.

Sprinkle the shredded mozzarella and grated Parmesan cheese evenly over the top. Bake, uncovered, for 20-25 minutes, or until the cheese is melted, bubbly, and lightly golden brown on top.

Remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows it to set up slightly. Garnish with fresh chopped parsley before serving.