Preheat your oven to the high grill/broil setting, positioning the rack in the middle.

In a small saucepan, combine unsalted butter, minced garlic, and a pinch of salt over low heat. Stir gently until the butter is fully melted and the garlic becomes fragrant, about 3–4 minutes.

Carefully clean the portobello mushrooms under cold water, removing any dirt. Apply a generous layer of the garlic butter mixture to the bottom sides of the mushrooms.

Flip the portobello caps over so the gill side is facing up. Brush the insides of each mushroom with the leftover garlic butter.

Layer slices of fresh mozzarella inside each mushroom, followed by a handful of halved grape tomatoes.

Place the filled portobellos on a baking sheet and slide them into the preheated oven. Grill/broil for about 8 minutes.

Once out of the oven, sprinkle fresh basil leaves on top and drizzle with balsamic glaze.