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. Flip and brush any remaining garlic over the insides of each cap. 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 the balsamic glaze and sprinkle with salt to taste. (If making from scratch, prepare while mushrooms are in the oven.) Combine sugar (if using) and vinegar in a small saucepan over high heat and bring to the boil. Reduce heat to low; allow to simmer for 5-8 minutes or until mixture has thickened and reduced to a glaze. (If not using sugar, allow to reduce for 12-15 minutes on low heat).