Toast bread in 350 degree F oven until golden brown, about 10 minutes. Rub lightly with garlic clove while still warm.
Remove gills from mushrooms with spoon and cut into 1/4-inch thick slices. In a large skillet heat 2 tablespoons oil over moderate heat until hot but not smoking and cook mushrooms until softened, then add garlic stirring, until all liquid mushrooms give off is evaporated, about 5 minutes. Remove skillet from heat. Keep mushrooms warm, covered.
Top toasted bread with sauteed mushrooms and tomato relish. Garnish with basil.
Cut tomatoes in quarters, combine red onion, basil, vinegar, olive oil. Season, to taste, and let marinate for 15 minutes.