Heat butter in a large frying pan. Add shallots, leek and garlic, then gently fry for 7 mins until softened.
Increase the heat, add the chestnut and the shiitake mushrooms, then cook for 10 mins, stirring, until the juices have evaporated and the mushrooms are tender. Stir in the mustard and crème fraîche, then season well. Cook for a further 2 mins then stir in the chopped tarragon.
Preheat the grill. Cut the French stick into diagonal slices, drizzle with a little olive oil, then grill until golden. Spoon the hot pâté on to the toasts, garnish with the extra tarragon, then serve with salad leaves.
Recipe from Good Food Vegetarian Christmas magazine, December 2006