In a skillet over medium heat, add olive oil and sauté the diced onion and bell pepper until soft, about 5 minutes. Add minced garlic and cook for another minute until fragrant.
Transfer the onion, bell pepper, and garlic mixture into the slow cooker.
Add in the pumpkin puree, black beans, kidney beans, diced tomatoes (with juices), vegetable broth, chili powder, cumin, smoked paprika, cayenne pepper, salt, and pepper.
Stir all the ingredients together until well combined.
Cover the slow cooker and set it to low heat for 6 to 8 hours or high heat for 3 to 4 hours, until the chili is heated through and the flavors meld.
Taste and adjust seasoning, adding more salt, pepper, or cayenne if desired.
Serve hot, garnished with fresh cilantro and a dollop of sour cream or Greek yogurt if desired.
Notes
Adjust the cayenne pepper to your preferred spice level.