Rinse and soak the lentils in a large bowl of water while preparing the other ingredients. Dice the onions, garlic, and ginger. Peel and chop the sweet potatoes into 1-inch cubes.
Add a little bit of the broth and use it to deglaze the bottom of the pan by scraping at the bits that are stuck to it. Pour in the remaining broth, diced tomatoes, sugar, and cinnamon stick.
Drain the lentils and stir them into the pot. Bring this to a boil, then turn the heat down to medium. Stir occasionally as it cooks for about 15 minutes or until the lentils have started to soften.