Heat the oil over medium heat in a large, deep pot.
Add the onion and cook, stirring frequently, for 2 minutes or until the onion starts to soften.
Stir in the tomatoes and ginger and cook for 3 minutes.
Stir in the turmeric, cumin, coriander, cinnamon, cayenne, and salt. Cook and stir for 2 minutes, then taste for seasonings.
Add the sweet potatoes, broth, and lentils. Stir well, and bring to a boil over high heat.
When the mixture comes to a boil, reduce the heat, cover and simmer for 40 minutes or until the lentils and sweet potatoes are soft.