Bring a large pot to medium heat and add a splash of water or neutral cooking oil. Add the onion to the pot and sauté for 3 to 5 minutes, until translucent. Add the ginger and garlic to the pot and cook for an additional 1 to 2 minutes, until fragrant. Stir in the curry powder, cumin, and cayenne and cook for 1 to 2 minutes more.
Stir the tomato paste and peanut butter into the mixture; once dissolved, add the diced tomatoes, sweet potatoes, red lentils, and vegetable broth. Bring to a boil over high heat, then cover and simmer over low-medium heat for 15-20 minutes, until the lentils have fully cooked.
Ladle into bowls and top with fresh cilantro; serve warm. Store any leftovers in the fridge for up to one week, or the freezer for up to two months.
