Add all the ingredients (except the garnish ingredients) into a large slow cooker and stir to combine.
Cook on low for 6 to 8 hours (or 4 to 5 hours on high), until the lentils are tender. If the chili isn’t thick enough for your liking, stir in additional tomato paste during the last hour.
Garnish each bowl with the lime zest and juice, cilantro, green onions, sliced avocado, and vegan sour cream, if desired.