In a saucepan over low heat, melt the butter. Saute the garlic and mushrooms until tender. Mix in the Brie. Cook and stir until melted and well blended.
Remove the top of the sourdough bread, and hollow out the center. Spoon the Brie mixture into the bread. Sprinkle with Parmesan cheese.
Place the filled bread on a baking sheet, and broil 10 minutes in the preheated oven, or until the top is lightly browned. Cool slightly, and slice into wedges to serve.
