Mix all the ingredients for the marinade in a bowl. Smear the marinade onto the rack of lamb, then cover with foil and refrigerate for at least 2 hours.
Remove from the fridge and rest at room temperature for half an hour. Meanwhile, preheat the oven to 200C/400F/Gas Mark 6.
Roast the marinated lamb for 25 minutes. Uncover and baste with the marinade. Cover and cook for a further 10 minutes.
Add a pinch of salt to the cold water and soak the grated potato. Leave for 5 minutes, then remove the potato from the water and squeeze out any excess moisture using a kitchen towel.
Sprinkle the remaining salt on the onions. Leave for 5 minutes.
When the oil is hot, add the potato and fry until golden brown. Remove from the pan, drain on kitchen towels and repeat the process with the salted onions.
Heat the oil for tempering in a frying pan. Add the cumin seeds and when they begin to crackle, add the sliced garlic. Cook until lightly brown.
Pour this over the cooked lamb and immediately garnish with the fried onions and potato straws. Serve with naan bread and a cool pomegranate and red onion raita.