Begin by preparing the marinade for the lamb. Add the yoghurt to a bowl or resealable bag large enough for the lamb and add the ground cumin, ground coriander, cinnamon, cardamom, turmeric, cayenne pepper, garlic, green chilli, lemon juice and a generous pinch of salt. Mix until combined.
Using a sharp knife, make slits all over the lamb to help the marinade penetrate the meat further. Rub the yoghurt mixture all over the lamb then leave in the fridge to marinade for at least 3 hours, but ideally overnight.
Once ready to cook, preheat the oven to 220C/200 fan/gas 7.
Place the marinated lamb in a roasting tin, over a rack, and place in the oven for 20 minutes.
After 20 minutes, decrease the heat to heat to 190C/170C fan/gas 5 and continue to cook for around 15-20 minutes per 500g of lamb. If you have a probe thermometer, you can also check the internal temperature of the roast lamb. At 55C the lamb will be medium-rare or well done at 70C. If the lamb begins to brown too quickly, cover with foil to prevent the crust from burning.
Once cooked remove from the oven and rest for 20-30 minutes loosely covered with tin foil.
While the lamb cooks, prepare your potatoes by peeling and cutting into thick slices. Add to a pan of boiling water alongside ½ tsp turmeric and a generous pinch of salt. Par-boil for 10 minutes, until fork tender.
As the potatoes boil, pour a generous drizzle of oil into a roasting tin and place in the oven to heat.
Drain and leave to dry completely before transferring to a large bowl. Pour in the flour, cayenne pepper, garam masala, ground coriander and ground cumin, then mix until well combined.
Remove the roasting tin from the oven and add the black mustard seeds and cumin seeds to the hot oil. Stir gently, then carefully pour in the potatoes. Season with salt then place in the oven with the lamb. As soon as the lamb comes out of the oven, increase the temperature to 200C/180 fan/gas 6. Cook for around 40 minutes in total, turning occasionally, or until cooked to your liking.
Once the potatoes are cooked, squeeze over some lemon juice and garnish with chopped coriander.