Chop onion and chicken into 1" pieces. In a large bowl, toss onion, chicken, tomatoes, olive oil, and seasoning until evenly combined.
Thread chicken and onion onto two Grill Boss Skewers and tomatoes onto a third skewer.
Grill skewers with lid closed, 3–4 min per side, or until chicken is cooked through. Remove tomato skewer if it starts to burst before chicken is cooked.
Arrange greens on a platter and slide on food from skewers. Cover with cheese and additional toppings, if desired.