If you are making horse gram rasam, you will already have cooked horse gram. If not, soak 1 cup horse gram in warm water for 2 hours. Pressure cook with 2 cups of water for 6-8 whistles.
In a kadai/pan, add in 1 tsp oil. Temper with 1 tsp mustard seeds, a big pinch of asafoetida, few curry leaves, 3-4 spicy dry red chillies, 2 large onions finely chopped. Fry until onions cook well but remain crunchy.
Next, add in the cooked horse gram, salt to taste, 3 to 4 tbsp (or more) fresh grated coconut. Mix everything well.
Lastly, squeeze half a lemon, add in chopped coriander leaves. Mix well and it is ready to be served.