Place the dried porcini in a small bowl. Pour over 75ml boiling water and set aside to soak.
Place the rice in a pan and cover with 400ml boiling water. Add a pinch of salt. Cover and simmer over a medium heat for 8 mins till the water is absorbed. Without removing the lid, take off the heat and set aside to steam in the pan for a further 5-10 mins. The rice will keep warm in the pan.
While the rice cooks, peel and crush or finely chop the garlic. Peel the ginger and cut into thin sticks. Slice the portobello mushrooms. Trim the beans and halve them.
Heat 1 tbsp oil in a wok or large frying pan for 1 min. Add the beef and stir fry for 2-3 mins till browned. Add the garlic and ginger. Stir fry for 30 secs, then remove everything from the pan.
Add 1 tsp oil to the pan. Add the portobello mushrooms. Stir fry for 4-5 mins over a high heat till softened.
Drain the porcini, reserving the liquor. Add the porcini to the pan and stir fry for a further 2 mins. Add the beans and stir fry for 1 min, then add the reserved porcini liquor. Cook for 1 min till the beans are almost tender – they should still have some bite. Return the beef and warm through for 2-3 mins.
Remove the wok from the heat. Drizzle in 2 tbsp vinegar. Taste and season with salt and pepper. Fluff the rice. Serve the stir-fry in warm bowls with the rice.