Put the stock, soy sauce and mirin into a saucepan and bring to a very gentle simmer.
Meanwhile, heat 2 tablespoons of the sunflower oil and the sesame oil in a separate saucepan, add the spring onions, garlic and ginger and cook over a high heat, stirring, for 1 minute. Add the rice and lime leaves and cook over a low heat, stirring, for 1 minute until all the grains are glossy.
Stir about 150 ml (¼ pint) of the stock into the rice. Cook over a medium heat, stirring constantly, until absorbed. Continue to add the stock, a little at a time, and cook, stirring constantly, until all but a ladleful of stock has been absorbed.
While the risotto is cooking, wipe the mushrooms, discard the stalks and thinly slice all but a few. Heat the remaining oil in a frying pan, add all the mushrooms and cook over a medium heat, stirring frequently, for 5 minutes until golden.
Add the coriander to the risotto with the sliced mushrooms and the remaining stock. Cook, stirring, until the stock has been absorbed and the rice is al dente. Serve garnished with the reserved whole mushrooms and coriander sprigs.