Place oil in a small saucepan over medium-high heat. When shimmering hot, sprinkle asafoetida (if using) over the oil, then add the mustard seeds and curry leaves. As soon as the mustard seeds pop and the curry leaves turn a shade darker, reduce heat to low and stir in the ginger and chiles. Fry for about 15 seconds, then add the tomatoes. Give it a stir, then stir it into the cooked lentils, oil and all. Taste and add more salt if needed (I added an extra 1/2 tsp).