Fry the onion in a spray of oil for about 5 minutes, until soft.Add the garlic and mushrooms and fry for another 5 minutes, with another spray of oil, until the mushrooms are golden and the liquid has been released from them and cooked off. Add the crème fraîche, a splash of water (or stock) and tarragon, simmer for 1 minute, season and serve with cooked grains.