Preheat the oven to 400° F. Line a baking sheet with a silicone baking mat (or parchment paper or aluminum foil), then spread 3 tablespoons of oil over it then set aside.
Remove the stems from your button mushrooms and make sure there is plenty of room for the caprese filling.
In a small bowl, add in tomatoes, basil, olive oil, a pinch of sea salt and pepper and toss together. Set aside for 10 minutes to marinate.
Fill each button mushroom with 1 tablespoon of cheese.
Top with 2-4 tomato halves each then place mushrooms on your prepared baking pan.
Bake for 20-25 minutes, or until cheese is a nice golden color. Remove from oven, sprinkle with extra basil and drizzle with balsamic vinegar, then serve your Caprese Stuffed Mushroom appetizers hot.