Preheat the oven to broil.
Line a baking sheet with aluminum foil and set aside.
In a small bowl, toss the mushroom caps with the olive oil until well coated. Use your fingertips to rub the oil in without breaking the mushrooms.
Place the mushrooms on the baking sheet gill-side down and broil the mushrooms until they are tender on the tops, about 2 minutes.
Flip the mushrooms over and broil 1 minute more.
Take the baking sheet out and spread the minced garlic evenly over the gill side of each mushroom.
Top each cap with a single tomato slice, sprinkle with the fresh basil, and finish by piling the shredded mozzarella cheese on top. Broil the mushrooms again until the cheese is beautifully melted and bubbly, about 1 minute.
Serve immediately.