Preheat your oven to grill/broil settings on high heat. This step is crucial as it creates a direct heat source that will give your mushrooms that perfect charred finish.
Adjust your oven shelf to the middle position. This ensures that your mushrooms will cook evenly without burning.
In a small saucepan or microwave-safe bowl, combine the butter, garlic, and fresh parsley. Melt the mixture until the garlic becomes fragrant, which should take about a minute in the microwave or a few minutes on low heat on the stove.
Brush the bottoms of each Portobello mushroom with the melted garlic butter mixture and place them, buttered side down, on a baking tray. This adds flavor directly to the mushrooms.
Carefully flip the Portobellos over, and brush any remaining garlic butter over the insides of each cap, ensuring they are well coated.
Now it’s time to fill the Portobello mushrooms. Layer the mozzarella slices and tomato slices generously inside each mushroom cap. Make sure to pack them in for a hearty bite.
Place the filled mushrooms under the broiler and grill until the cheese has melted and turns golden, which should take about 8 minutes. Keep an eye on them to prevent burning.
Once done, remove the mushrooms from the oven and top them with shredded basil. This fresh touch enhances the flavor.
Drizzle the filled mushrooms with balsamic vinegar and sprinkle with salt to taste. The balsamic adds a tangy contrast that elevates the dish.
If you decided to use brown sugar, combine it with the balsamic vinegar in a small saucepan over high heat. Bring it to a boil, then reduce to low and simmer for about 5 to 8 minutes until it thickens into a glaze. If you’re not using sugar, allow the vinegar to reduce for 12 to 15 minutes on low heat until syrupy.