Heat a grill on high heat. For rarebit, combine ingredients in a bowl, season to taste.
Heat butter in a large frying pan over high heat, add mushrooms and thyme and stir occasionally until mushrooms are golden (3-5 minutes). Season to taste and set aside.
Meanwhile, place bread slices on an oven tray and toast, turning once, until golden (1-2 minutes each side). Top with mushroom mixture and crumbled rarebit mixture. Grill on centre shelf of oven until golden and melted (3-5 minutes). Serve hot with radicchio salad.
This recipe is from the August 2010 issue of Australian Gourmet Traveller.