Heat oil in large stockpot over medium high heat. Add onions, garlic, and bell peppers and cook for five minutes. Add beef and cook, breaking up with back of spoon, until browned. Add 3/4 teaspoon each salt and pepper.
Add chili powder and saute for 30 seconds. Stir in green chilis and tomatoes, with their juices. Bring to a boil, reduce heat and simmer, covered for 10 minutes. Add pumpkin and cranberries and cook for 10 minutes more.
Ladle chili into six bowls. Top with shredded cheddar cheese and chopped cilantro and green onions, if desired.