Tip the rice into a large bowl. Cover with cold water. Whisk for 1-2 mins (you can use a fork) till the water goes cloudy. Drain, rinse under cold water and tip back into the bowl. Cover with fresh cold water. Set aside to soak.
Peel and finely chop the onion. Finely chop the tomatoes into small dice. Trim the dry ends off the rhubarb sticks and chop them into chunks around 2cm long.
Warm a medium pan over a medium heat for 1 min. Add ½ tbsp olive oil and the lamb. Fry for 5 mins, turning once or twice, till the lamb is browned all over. Lift the lamb out of the pan and pop onto a plate.
Add the onion and tomatoes to the pan with 2 tbsp water. Season with salt and pepper. Cook and stir for 5 mins till the onion and tomatoes are soft and look glossy. Slide the lamb back into the pan with any juices from the plate.
Add 6 cardamom pods to the pan with 1 tsp cumin seeds and 1 cinnamon stick. Pour in 400ml boiling water. Cover. Bring to the boil, then turn the heat down and simmer for 25 mins. Add the rhubarb to the pan with 1 tsp Demerara sugar and simmer for another 5 mins.
When the korma has been cooking for 20 mins, drain the rice. Tip it into a small pan. Pour in 400ml boiling water and add a pinch of salt. Cover, bring to the boil and then turn the heat right down. Simmer for 8 mins till all the water is absorbed and the rice is tender. Take off the heat and set aside, lid on, for 2 mins.
Fish the cinnamon stick and cardamom pods out of the lamb korma (if you can spot them). Taste and add more salt, pepper or sugar if you think it needs it. Serve the rice with the korma, garnished with torn mint leaves.