Heat the oil in a large frying pan over a high heat. Reduce the heat to medium, add the onions and fry, stirring, for 2 minutes. Add the garlic and rosemary and fry for a further 1–3 minutes until the onions are softened. Add the chestnut mushrooms, sprinkle with salt and continue to fry until all the liquid is absorbed.
Put the porcini mushrooms in a sieve and rinse under cold running