Put ½ the thumb-sized piece of ginger, 2 quartered onions and 4 garlic cloves in a food processor with 300ml water. Blitz to a smooth purée.
Scrape down the sides with a spoon and blitz again to make it as smooth as you can. Tip into a deep sauté pan, cover with a lid and simmer for 15 mins.
Remove the lid and cook for 5 mins more, stirring occasionally. By now the liquid should be all gone. If not, cook a little longer.
Add 2 tbsp rapeseed oil to the pan with the remaining piece of ginger, cut into matchsticks. Turn up the heat and fry, stirring, for 3-5 mins until it starts to colour.
Stir in 1 cinnamon stick, 1 tbsp ground coriander, 1 tsp ground cumin, 1 tsp ground turmeric and ½ tsp fennel seeds, then add 750g leg of lamb. Stir-fry until the lamb changes colour.
Tip in 400g can chopped tomatoes with a can of water and 1 deseeded and sliced red or green chilli, season well, cover and leave to simmer for 1 hr.
Stir in the finely chopped stalks from a small bunch coriander, re-cover and cook for a final 30 mins until the lamb is tender. Add a splash of water if necessary to loosen the consistency as it cooks.
Stir in the roughly chopped coriander leaves and serve with basmati rice and mango chutney or raita.