Remove mushroom stems and discard. Place mushrooms, top side down into a shallow plate or serving dish. Set aside.
In a separate bowl, add oil, chopped onion, minced garlic and balsamic vinegar and mix all ingredients well.
Spoon 1/3 of the mixture into each of the mushroom caps.
Let Portobello mushrooms stand 1 hour to fully marinate.
Heat a grill on medium heat, typically around 400 degrees.
Cook mushrooms until hot, about 10 minutes. Please note that if you cook both sides (5 min a piece) that flipping them will result in some of the ingredients fall into out, but also give a nice grill flavor on both sides. Please see full article about deciding which sides to grill and suggestions.
Remove with tongs and serve mushrooms immediately topped with diced tomatoes and chopped fresh parsley or cilantro.