Bring a large pot of salted water to a boil. Add penne pasta and cook until al dente, about 8-10 minutes. Drain and set aside.

In a skillet, heat olive oil over medium heat. Add diced chicken, season with salt and pepper. Cook until browned and cooked through, about 6-8 minutes.

Add fresh spinach to the skillet and cook for 2-3 minutes until wilted.

In a bowl, combine basil pesto and heavy cream. Stir in the cooked chicken, wilted spinach, and drained pasta. Mix in ¾ cup mozzarella cheese and red pepper flakes if using.

Preheat oven to 350°F. Transfer the pasta mixture to a greased baking dish. Top with remaining mozzarella and Parmesan cheese.

Bake for 20-25 minutes until cheese is bubbly and golden brown. Let sit for a few minutes before serving.

Garnish with fresh basil leaves before serving. Enjoy with a side salad or garlic bread.