1. In a large pot, heat the oil over medium heat and saute the onion, bell pepper, and garlic until tender (8 - 10 minutes).
2. Stir in the tomato puree, salt, mustard, maple syrup, water, chili powder, and lentils. Bring to a rolling boil.
3. Lower the heat to a simmer, cover, and cook until the lentils are tender (20-25 minutes). Stir regularly to prevent sticking.
4. Slice the sweet potatoes in half lengthwise, mash with a fork, and top with lentils. Serve hot.