Grilled Portabellas

Preheat outdoor grill or broiler. Seperate mushroom caps from stems, set caps aside. Coarsely chop stems (to measure 1 cup). In a medium saucepan melt butter. Add chopped stems and garlic, cook, stirring frequently, until stems are tender, 5 to 6 minutes. Stir in vinegar, salt and black pepper, remove from heat. Transfer to the bowl of a food processor, process until finely chopped, set aside. Place mushrooom caps, rounded side up, on a grill or rack in a broiler pan. Grill or broil 4 inches from heat until mushrooms are hot, 2 to 3 minutes. Turn, spread each mushroom with 1/4 of the chopped mushroom stem mixture, cook until mushrooms are tender, 4 to 5 minutes longer. Serve on taosted rolls with sliced tomato and arugula or watercress, if desired. Yield: 4 portions