Strain lentils and add to the pot, along with water and tomatoes. Give a stir, bring to boil, cover, and turn heat down, and let gently simmer, about 15 minutes. After 15 minutes, uncover and check consistency, If watery, continue to simmer uncovered until it reduces and becomes thicker.
Saute onion for two minutes, stirring, then add ginger. Saute two more minutes, then add garlic and turn heat to medium. Add spices, salt, sugar, bay leaves and chili. Stir for another minute letting spices toast, then add sweet potatoes.
In a large heavy bottom pot or Dutch over, heat oil over medium high heat.