Garnish: finely chopped fresh chives *available at some specialty foods shops and by mail order from Marché aux Delices, tel. (888) 547-5471 Chop oyster mushrooms and finely chop onion. Mince garlic. In a 4- to 5-quart heavy kettle heat oil over moderately high heat until hot but not smoking and sauté mushrooms, onion, and garlic with porcini powder and brown sugar, stirring, until liquid mushrooms give off is completely evaporated and mushrooms begin to brown. Add Madeira and boil, stirring occasionally, until liquid is evaporated. Stir in mushroom stock, vinegar, and thyme and bring to a boil. Stir in cream and salt and pepper to taste and simmer, uncovered, 15 minutes. In a blender purée soup in batches until smooth (use caution when blending hot liquids), transferring to a saucepan, and reheat over moderately low heat until hot. Serve soup garnished with chives. Makes about 5 cups.