Slice the shiitake mushrooms, or, if using dried mushrooms, cover them in boiling water and leave for 20 minutes to rehydrate. Lift out the mushrooms, strain the liquid through a sieve lined with kitchen paper and set it aside. Slice the mushrooms. Chop the oyster and field mushrooms into large chunks.
Heat the oil in a large saucepan, add the onions and fry over a high heat for 5 minutes, or until they start to brown. Add the mushrooms and continue frying for a further 1-2 minutes.
Add the sweet potatoes, soy sauce, stock or water, replacing some of the stock with the mushroom liquid if you used dried mushrooms. Bring to the boil, reduce the heat to low and simmer, uncovered, for 30-40 minutes until the sweet potatoes have softened and most of the liquid has evaporated. Stir occasionally during cooking.
Season to taste, stir in the coriander and serve.
Variation: this recipe works equally well with yellow or white-fleshed sweet potatoes, although the orange sweet potato is the most widely available variety.