Preheat oven to grill/broil settings on high heat. Arrange oven shelf to the middle of your oven.

Combine all of the Garlic Butter ingredients together in a small saucepan (or microwave safe bowl), and melt until garlic is fragrant. Brush the bottoms of each mushroom and place them, buttered side down, on a baking tray.

Fill each mushroom with the mozzarella slices and tomatoes, and grill/broil until cheese has melted and golden in colour (about 8 minutes).

To serve, top with the basil, drizzle with balsamic vinegar and sprinkle with salt to taste.