Rub chicken evenly with the 1/2 the fajita seasoning. Heat the oil in a skillet over medium heat, and cook the chicken 8 minutes on each side, or until thermometer inserted reaches 165°; slice and set aside.
In a large saucepan, mix beans, corn salsa and other 1/2 of fajita seasoning. Heat over medium heat until warm.
Prepare the salad by tossing the greens, onion and tomato. Top salad with chicken and dress with the bean and corn mixture. (If you are preparing ahead of time, combine the chicken with the corn and bean mixture and store separate from the salad in the fridge.)