Heat oil in a kadhai. Once the oil is hot. Add mustard seeds, cumin seeds, chopped green chilli and few curry leaves. Let it crackle for a few minutes. Then add boiled potato. Cut into cubes or mashed it. Fry it for few minutes. Then add turmeric powder, red chilli powder, roasted cumin powder, 1 tbsp crushed ginger, chopped green chilli, garam masala (optional) and black salt as needed. Fry till the masala coated well.