Heat the oil over medium heat.

Once the oil is hot, add the onion and garlic.

Cook, stirring often, until the onions are translucent, about 6 minutes.Stirring constantly, add the water, lentils, spices and salt. Bring to a low boil, then turn down the heat to low, cover and let the soup simmer for about 20 minutes, or until lentils are very tender.

Stir in the tomato until well combined. Cook several minutes more, or until the soup is desired temperature and consistency, adding more water to the dahl if needed.