Alright so what you need is 3 large portobello mushrooms, ¼ cup canola oil (or oil of your choice), ¼ cup balsamic vinegar, 3 tablespoons chopped onion, and 4 minced cloves garlic.
First Clean the mushrooms; remove stems, reserving them for another use. Place mushroom caps gill-side up in a shallow dish.
Then Combine oil, balsamic vinegar, onion, and garlic in a small bowl. Pour mixture evenly over mushroom caps; let marinate at room temperature for 1 hour.
Go and Preheat the grill to medium-high heat; grease the grate.
Lastly, Grill those suckers over the hot grill until caramelized and tender, about 5 minutes per side; serve warm.