Preheat your oven to 375°F (190°C).
In a large skillet, heat olive oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until it becomes translucent.
Stir in the minced garlic and cook for an additional minute, until fragrant.
In a large bowl, combine the shredded chicken, pumpkin puree, cooked rice, chicken broth, pumpkin pie spice, salt, and pepper, mixing until well combined.
Fold the sautéed onion and garlic into the mixture, ensuring an even distribution.
Transfer the mixture into a greased 9x13-inch baking dish, spreading it out evenly.
Sprinkle the shredded cheddar cheese generously over the top.
Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden brown.
Let the casserole cool for 5 minutes before serving.
Garnish with fresh herbs if desired, and enjoy your cozy family meal!