Set the Instant Pot to Sauté mode. Add the olive oil, onion, and red bell pepper. Cook for 4 to 5 minutes until they begin to soften.
Add the garlic, cumin, smoked paprika, chili powder, and chipotle. Cook for about 30 seconds.
Pour in the vegetable broth. Scrape the bottom of the inner pot well with a wooden spoon to remove any stuck bits.
Add the rinsed red lentils and fire roasted tomatoes. Do not stir after this point.
Spoon the tomato paste over the ingredients. Add the pumpkin puree as the final layer and gently spread it across the top. Do not stir.
Close the lid and set the Pressure Release Valve to sealing. Cook on High Pressure for 10 minutes.
When the cooking cycle ends, follow the pressure release directions for your Instant Pot model. Open the lid only when the pressure has fully dropped.
Stir the pumpkin into the chili. Add the rinsed canned black beans.
Select Sauté mode and cook for about 5 minutes, stirring from time to time, until the beans are hot and the chili reaches your preferred thickness.
Turn off Sauté mode. Stir in the lime juice and apple cider vinegar. Season with salt and black pepper.
Fold in toasted walnuts if using. Serve hot with avocado, cilantro, jalapeño slices, lime wedges, or your favorite chili toppings.
