Pop the porcini in a heatproof bowl. Pour in 500 ml boiling water. Leave to soak. Tip the rice into a large bowl and cover with cold water. Whisk for 1-2 mins (you can use a fork) till the water goes cloudy (this is the starch). Drain and rinse again.
Tip the rice into a small pan. Add 400 ml boiling water and a pinch of salt. Cover, bring to the boil. Turn the heat down to very low. Gently cook for 25 mins till the water is absorbed. Take off the heat and remove the lid. Lay a clean tea towel over the pan and put the lid back on. Set aside to finish cooking.
While the rice cooks, trim the very top and bottom off the leek. Halve it and rinse it, then slice it. Break the broccoli into small florets. Slice the mushrooms. Peel and grate the ginger and garlic. Drain the tofu. Chop into bite-sized pieces.
Warm 1 tbsp oil in a deep frying pan or wok. Add the leek and stir fry over a medium heat for 2-3 mins so they just soften. Add the fresh mushrooms. Stir fry for 4-5 mins till they’re juicy. Add 2 tbsp of the tamari and the vinegar.
Drain the porcini (keep the soaking liquid for later). Add the porcini to the wok, along with the ginger and garlic. Stir fry for 2 mins till they’re aromatic.
Mix the cornflour with 1 tbsp boiling water in a jug to make a smooth paste. Stir into the wok. Pour in the porcini water. Add the broccoli and tofu. Cover and simmer for 10-12 mins, stirring occasionally, till the veg are tender.
Taste the hotpot. Add any remaining tamari if it needs it. Serve the rice and hotpot in warm bowls.