Separate the chard stalks from the leaves, cutting both into uniform pieces. Pop the mustard seeds in hot oil, then fry the onions for 12 to 15 minutes until soft and golden brown.

Stir in the ginger and garlic, followed by the potato cubes and a splash of water. Cover and steam the potatoes for 10 minutes until they begin to soften.

Add the tomatoes and chard stalks, simmering covered for 5 minutes. Stir in the dry spices and salt, then finish by folding in the chard leaves until they are just wilted.