Heat the vegetable oil in a large pan over medium heat.

Add the cumin seeds and mustard seeds to the hot oil, and let them sizzle for about 30 seconds until they begin to crackle.

Add the sliced onions and sauté until they turn golden brown.

Stir in the turmeric powder, coriander powder, and red chilli powder. Cook for 1 minute to let the spices release their aroma.

Add the diced potatoes to the pan, stirring well to coat them with the spices.

Add salt to taste, and stir in the green chilli.

Pour in 200 ml of water, stirring to combine.

Cover the pan and cook on low heat for 15-20 minutes, stirring occasionally, until the potatoes are tender and the water has mostly evaporated.

Once the potatoes are cooked, sprinkle the garam masala and amchur powder over the dish. Stir well to combine.

Garnish with fresh coriander before serving.