Heat a large skillet over medium-high heat and add the ground beef, breaking it apart with a spatula. Cook for about 5 minutes until crumbly and no longer pink.

Add the diced onion, minced garlic, and crushed red pepper flakes to the skillet with the beef. Cook until the beef is fully browned and the onion softens.

Toss in the chopped green and red bell peppers and cook for an additional 5 minutes until tender.

In a large slow cooker, combine the petite diced tomatoes, kidney beans, black beans, Great Northern beans, pumpkin puree, fire-roasted tomatoes, and both tomato sauces.

Season the mixture with pumpkin pie spice, chili powder, ground cumin, and salt.

Stir in the cooked beef and vegetable mixture from the skillet into the slow cooker.

Cover and cook on the Low setting for 1 to 2 hours.

Serve the chili in bowls, garnished with toppings like sour cream, shredded cheese, avocado slices, or chopped green onions.