Meanwhile, melt ghee in a large skillet over medium heat. Stir in onions and garlic. Cook and stir until the onions are soft and beginning to brown, about 10 minutes. Add the coriander, cumin, cayenne pepper, and cilantro. Cook and stir for 2 minutes more, then set aside. Once the lentils are done, discard the ginger slices, stir in the onion mixture and ladle into bowls to serve.