Combine the lamb with all the marinade ingredients and mix well. Cover and allow to marinate for at least an hour or overnight in the fridge.
To make the curry, soak the cashews in boiling water for 10 minutes then blend until smooth.
In a large pot, heat the ghee/butter and add the onions. Cook until the onions are soft and translucent then add the garlic, ginger and spices. Cook for a minute or two until the pot is dry and the mixture is aromatic.
Add the lamb and all its marinade, stock/water, cashew paste and a pinch of sugar. Bring to a simmer then reduce the heat, cover and simmer for 1-2 hour until the lamb is tender and the sauce has reduced. Stir the sauce every 20-30 minutes and top up with more stock or water if the sauce is getting too thick.
Season with salt, pepper and a squeeze of lemon then serve with your choice of side dishes.