Stir 2 tbsp Demerara sugar, the tamari and tamarind together in a dish. Drain the tofu and chop it into bite-size chunks. Add it to the tamarind marinade. Turn to coat and set aside for later.
Tip the rice into a bowl. Cover with cold water.Whisk (you can use a fork) for 1-2 mins till the water goes cloudy – this is the starch. Drain the rice and rinse it under cold water. Tip it into a small pan. Pour in 400 ml boiling water. Add a pinch of salt.
Put a lid on the pan. Bring to the boil, then put the rice on the lowest heat possible and gently cook for 25 mins till the water is absorbed. Turn off the heat and let the rice sit in the pan, lid on, to steam for 5 more mins. It will keep warm in the pan.
While the rice cooks, prepare your veg. Trim the very top and bottom off the leek. Halve it and rinse out any grit. Finely slice it. Trim and peel the carrots. Thinly slice them on the diagonal. Finely chop half of the cabbage into thin shreds (see our tip for what to do with the rest).
Peel the mango. Slice as much of the flesh off the stone as you can, then chop into long thin slices.
Put a wok or deep frying pan over a high heat and warm for 1 min. Add 1 tbsp olive or sunflower oil. Add the leek and carrots. Stir-fry for 5 mins till the veg start to soften and look a little glossy. Add the cabbage and stir-fry for 4-5 mins to just soften and wilt it.
Tip the tofu and its marinade into the pan. Add 200 ml boiling water and the mango. Simmer, without a lid on the pan, for 8-10 mins to cook everything through. Serve the stir-fry with the warm rice.