In another large pan, heat oil over medium-high heat. Once hot, add the cumin seeds and sauté for about 15 seconds. Add the onions and sauté until slightly golden, about 5-7 minutes. Add the garlic and ginger and continue to sauté until the raw smell disappears (about 30 seconds). Add the tomatoes, green chili, and dry spice powders (red chili, turmeric, cumin, coriander) except the garam masala. Cook until soft and you see the oil leaving the sides, about 3 minutes.
Stir the cooked dal into the tadka and add 2 cups of water. Stir well and raise the heat to bring to a simmer.
Lower the heat and add the salt. Simmer this dal, stirring occasionally, for at least 5 minutes or until you achieve the desired consistency.
Add in the ghee. Rub the fenugreek leaves, if using, between your hands to crush them a little and add this to the dal.
Turn off the heat. Add lemon juice, chopped cilantro, and garam masala, if using. Give it a stir.
Serve dal hot with rice, roti, naan or anything else your heart desires. I like to serve it with some sliced onions, carrots or cucumber on the side.