Boil the cassava in a pan of salted water for 10 to 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 to 30 seconds, or until the mustard seeds start to pop. Be careful to keep the pan well away from your eyes
and face as the mustard seeds
pop and splatter.
Add the fennel seeds, onion, and green chiles
for 4 to 5 minutes, or until the onion has softened, and is golden brown.
Add the curry leaves
and the garlic and simmer
for 45 to 60 seconds, or until just golden brown.
Add the garam masala
, milk (use less milk if you are using potatoes and more milk if you are using cassava), cream, and grated coconut
, and stir until well combined. Add the milk mixture gradually, as necessary until the mashed potato or cassava is smooth. Season the mixture, to taste, with salt, and freshly ground black pepper.
Bring the mixture to a boil, then stir it into the mash
and add the lemon juice
, to taste. Serve immediately.