Cut the stems off the mushrooms, either discard or grill alongside the caps. Gently rinse the Portobellos and set aside to drain off excess water. In the mean time heat the grill. Rub the Portobellos with a little bit of olive oil, salt, ground black pepper and granulated garlic to taste. Place on hot grill. Grill on one side for 10 minutes then flip over and grill for another 5 minutes. The grilling time will depend on how large and thick the Portobellos are. Once cooked they should be soft and tender.
While the mushrooms are grilling saute the green and yellow onions until soft and translucent. Season with a little salt and ground black pepper. Turn heat off and set aside.
Once the mushrooms are cooked through serve with sauteed onions and baked potato or any other vegetable as desired.