Preheat oven to 400°. In a small saucepan, heat oil over medium heat. Add garlic; cook and stir until tender, 1-2 minutes (do not allow to brown). Stir in vinegar; remove from heat., Place mushrooms in a 13x9-in. baking dish, gill side up. Brush with vinegar mixture; sprinkle with salt and pepper. Top with polenta and tomato slices; sprinkle with cheese., Bake, uncovered, until mushrooms are tender, 20-25 minutes. Sprinkle with basil.