Warm the oil in a dutch oven on medium heat, then add the onions, and garlic to sauté.
Once the onions have softened, add the raw lentils, diced tomatoes, sweet potato, cumin, and chili powder. Then add 3 3/4 cups of water, and bring it to a simmer.
Cook on medium-low heat until the lentils and sweet potatoes are tender, about 45 minutes.
Add the salt, and more spices if necessary. Serve over the cooked rice and garnished with fresh cilantro.