Heat oven to 375f and move oven rack to middle.
Clean portobello mushrooms with a dry paper towel to remove dirt. Remove gills and stem of each mushroom with a metal spoon. Place mushrooms on baking sheet.
Season each mushroom with half the olive oil, salt, and pepper and place mushroom tray in oven to bake. Set timer for 10 minutes.
Slice fresh mozzarella into ¼" pieces. Set aside.
Cut cherry tomatoes into quarters. Chop a ½ cup of red onion. Mince 3 cloves of garlic.
Combine the tomatoes, onion, and garlic in a mixing bowl. Add 2 Tbsp of olive oil and 2 Tbsp of balsamic vinegar. Add remaining salt and pepper to bowl and stir it all together.
Remove mushrooms at ten minute mark and dry off the excess moisture with a paper towel. Evenly distribute the bruschetta to each mushroom and top with one slice of the fresh mozzarella.
Drizzle a tiny bit of oil onto each mushroom and return to oven for 5 more minutes.
Chop up a ¼ cup of fresh parsley.
Remove mushrooms again at 5 minute mark. Move oven rack to highest setting. Turn broiler to high.
Place mushrooms in oven and broil. Watch that the cheese does not burn. 2-3 minutes should get the cheese nice and bubbly.
Remove and plate the Portobello mushrooms and top with the fresh parsley. Serve balsamic and extra virgin olive oil on the table for anyone who wants more of either. Enjoy!