Preheat the oven to gas 6, 200°C, fan 180°C. Heat 1 tablespoon of oil in a large, lidded saucepan over a medium heat and cook the onions, stirring, for 15-20 mins until really soft.

Heat a pan and add the oil and cumin and mustard seeds. When the seeds start to pop, add the asafoetida, garlic, tinned tomatoes and spices and cook for 5 minutes.

Add the potatoes, bring to the boil and let them simmer gently for 3-4 minutes. Add a little water if it seems too dry.

Stir in the lemon juice and garnish with chopped coriander just before serving.