Put the mushrooms in a bowl and cover with 1 cup of boiling water. Leave them to soak for 15 minutes, or until rehydrated and softened, then drain in a sieve with a double layer of paper towels, reserving the soaking liquid. Roughly chop the mushroom caps, discarding any tough stems.
Meanwhile, heat the oil in large saucepan over a moderate heat. Add the onion and garlic and cook for 8 minutes, stirring frequently, until the onion is golden.
Stir in the tomatoes, ginger, tarragon, chopped mushrooms and their soaking liquid and 2 cups of water. Add the lentils and bring to the boil. Reduce the heat, cover and simmer for 25–30 minutes, or until the lentils are tender. Season to taste with salt and pepper.