This is a beautiful autumn soup for when the nights start to get chilly. Farro is the grain of a specific species of wheat, but the name is sometimes used interchangeably for barley or spelt as well. Particularly popular in Italy, it is shaped like rice and has a nutty flavour. You should be able to find it in Italian delis, but if you can’t get it you could use pearl barley instead.
Likewise, if you can’t get the porcini mushrooms, use half chestnut mushrooms and half fresh shiitake mushrooms to give a similar taste and texture.
Bring a large pot of salted water to a boil. Add the farro and cook for around 18 minutes until al dente. Drain and spread on a large plate to cool.
Trim the porcini mushrooms and cut the stems from the caps. Dice the stems and slice the caps thickly.
Heat 2 tablespoons of the olive oil in a large pot over a medium high heat. Add the onion, celery and half of the garlic. Reduce the heat to medium and cook for about 10 minutes until softened, stirring frequently. Stir in the farro. Cook it for another minute and then add 2 tablespoons of the wine and cook until it has evaporated. Add the boiling water and tomato purée. Stir to combine, bring to the boil, and then take the pan off the heat and cover to keep it warm.
Heat 2 tablespoons of the olive oil in a large non-stick sauté pan over a medium-high heat. Add the mushroom caps and stems, the remaining garlic, the rosemary and a pinch of salt and pepper. Cook the mushrooms until they are golden. Add the mushroom stems to the soup and transfer the caps to a plate.
Put the remaining 2 tablespoons of oil in the sauté pan over a high heat. Throw in the mussels and the other 70ml of wine and season. Cover the pan and cook until the mussels just open. Transfer the opened mussels to a bowl and pour the liquid from the pan into the soup.
Remove the mussels from all but 18 of the shells. Stir the shelled mussels into the soup before heating it to a bare simmer. Whisk in the butter and check the seasoning. Serve the soup garnished with the remaining mussels in their shells, the reserved mushroom caps and some roughly chopped parsley.
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