If you're making the coutes, it's a good idea to get them done in advance and out of the way. You can fry them, but I think they're much nicer baked to a crisp golden crunchiness in a slow oven. Set the oven to 150 C (300 F, Gas Mark 2). Stamp circles in the bread with a large pastry cutter; spread on both sides with butter and put them on a baking sheet. Bake for 1 hour, or until completely crisp and golden. Cool. These will keep in a tin for a few days. To prepare the mushrooms, cut off any stalks so that the surface is level, then wash the mushrooms and pat them dry on kitchen paper. Fry them on both sides with olive oil and drain well. Season them with salt and pepper, then leave on one side while you make the stuffing. Melt the butter in a medium-large saucepan. Add the onion and fry for about 7 minutes, until soft. Chop up any pieces of mushroom stalk, add these and cook for a minute or two longer. Remove from the heat and add the cooked chestnuts, breaking them up a bit as you do so to make a mixture which holds together but has some chunky bits in it. Add a dash of fresh lemon juice, and salt, pepper and grated nutmeg to taste. To serve the dish, preset the oven to 200 C (400 F, Gas Mark 6), or preheat the grill to high. Put the croutes on a baking sheet or in a shallow casserole, then place a mushroom on each one, black side up. Spooon the stuffing mixture on top. Bake or grill until heated through -about 10 minutes under the grill, 15-20 minutes in the oven. [Note from Karen: There's a gorgeous photo of this dish in the book. The stuffed mushrooms are each garnished with three or four extremely thin slivers of lemon peel. Looks absolutely beautiful.]