Heat the oil in a soup pot. Add the onion and sauté over medium-low heat until translucent. Add the garlic and continue to sauté until both are golden.
Add the lentils and 6 cups water. Bring to a slow boil, then lower the heat.
Cover and simmer gently for 10 minutes.
Add the remaining ingredients except the cilantro and salt. There should be enough liquid in the pot to not quite cover the ingredients, so if need be, add a little more water.
Bring to a slow boil once again, then turn down the heat and continue to simmer gently until the lentils and sweet potatoes are tender, about 30 minutes.
Stir in the cilantro, then season with salt and pepper. If time allows, let the chili stand off the heat for an hour or more before serving to allow the flavors to meld. This step is highly recommended.
Just before serving, taste to see if you’d like to boost the seasonings, and add a little more water if need be, but let the texture remain nice and thick. Pass around desired garnishes for embellishing individual servings.