Heat the oil in a large saucepan over medium heat. Saute the onion until softened. Add the sweet potato and saute for another 5 minutes, then add ginger and garlic, stir, and reduce heat to low.

Add the chili, lentils, and spices to the pan. Stir until the lentils are well-coated with oil, then add tomatoes and half of the water. Raise heat to bring to a boil, then reduce heat until the mixture is at a fast simmer. Cook uncovered until the lentils and sweet potatoes are soft, stirring occasionally and adding water as needed (about 25 minutes).

Season to taste with salt and continue to simmer until the mixture has thickened. Whisk to combine the lentils and sweet potatoes. Serve with rice and garnish with cilantro.