Place chicken tenders in a large bowl. Add cumin, chili powder and salt and season chicken well.
Preheat gas grill to medium high. Place a large, cast iron skillet on the grill. Add oil to the skillet and then add onions. Sauté onions until caramelized, about 10 minutes.
Meanwhile, place the chicken tenders across the grill. Grill chicken until done, about 5-7 minutes. Carefully remove onions and chicken from grill and place in separate bowls. When chicken has cooled, dice and set aside.
Toss all of the cheeses in a large bowl. Set aside. Place four tortillas on the grill. Place about ¼ cup of cheese, about ¼ cup of onions and about ¼ of the chicken on each tortilla. Spread evenly. Top with another tortilla and cook for about 2 minutes or until the bottom tortilla browns and the cheese melts. Gently turn the quesadilla over and cook for an additional 2 minutes. Remove and cut into wedges. Serve with sour cream and cilantro.