In a small saucepan simmer tomatoes in water, uncovered, until tender, about 10 minutes. In a blender puree tomatoes, cooking liquid, and vinegar until smooth and season with salt and pepper. Cut mushrooms into 1/4-inch thick slices and mince garlic. In a large skillet heat 2 tablespoons oil over moderate heat until hot but not smoking and cook mushrooms and garlic with salt and pepper to taste, stirring, until liquid mushrooms give off is evaporated, about 5 minutes. Remove skillet from heat and toss mushrooms with remaining tablespoon oil. Keep mushrooms warm, covered.
Toast bread and spread with tomato puree. Top puree with mushrooms.
Recipe courtesy of Gourmet Magazine