In a small saucepan simmer tomatoes in water, uncovered, until tender, about 10 minutes. In a blender purée tomatoes, cooking liquid, and vinegar until smooth (use caution when blending hot liquids) 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 purée. Top purée with mushrooms.
