180-280ml/6-10fl oz milk (use less milk if using potatoes and more if using cassava)
salt and freshly ground black pepper
1-2 tsp lemon juice
Boil the cassava or potatoes in a pan of salted water for 10-15 minutes, or until tender. Drain well, then mash with a potato masher or ricer until smooth and set aside.
Meanwhile, heat the oil in a small pan over a medium heat. Add the mustard and cumin seeds and fry for 20-30 seconds, or until the mustard seeds start to pop. (CAUTION: keep the pan well away from your eyes and face.)
Add the fennel seeds, onion and green chillies. Fry for 4-5 minutes, or until the onion has softened and is golden-brown.
Add the curry leaves and the garlic and simmer for 45-60 seconds, or until just golden-brown.
Add the garam masala, milk, cream and grated coconut and stir until well combined. Add the milk gradually as necessary until the mashed potato or cassava is smooth. Season, to taste, with salt and freshly ground black pepper
Bring the mixture to the boil, then stir it into the mash and add the lemon juice, to taste. Serve immediately.