If using dried mushrooms, plump them in the stock for about 1 hour before cooking. In a large heavy saucepan, place the stock and the mushrooms over moderate heat. Bring to a simmer and continue to cook for about 15 minutes. Drain the mushrooms, reserving the stock. Strain the stock through a double thickness of dampened cheesecloth and return it to the saucepan. Coarsely chop the mushrooms in a food processor, pulsing 4 or 5 times. Do not puree; they should be chunky. Return the chopped mushrooms to the stock and add the cream. Bring to a simmer over moderately low heat and whisk in the flour-butter mixture. Season to taste with salt and pepper and cook for about 10 minutes, or until the flour dissolves. Serve immediately.