In a large pot, cook the ground beef and chopped yellow onion over medium heat until the beef is no longer pink. Drain off any excess fat.
Stir in the sliced mushrooms, undrained diced tomatoes, canned pumpkin, pumpkin pie spice, salt, black pepper, and chili powder. Bring the mixture to a boil, then reduce the heat to low and let it simmer uncovered for 1 hour, stirring occasionally. Serve hot.
Notes
This pumpkin chili is best served warm. Garnish with your favorite toppings like sour cream, shredded cheese, or chopped cilantro.