6 small boneless, skinless chicken breast halves (about 2 1/4 pounds)
1 cup coarsely chopped fresh oregano leaves (if you're using dried, use half the amount)
4 scallions, trimmed and thinly sliced
1/2 cup balsamic vinegar
1/3 cup extra virgin olive oil
2 teaspoons freshly ground black pepper
3/4 teaspoon salt
Place the chicken breast halves between 2 sheets of plastic wrap. Using a mallet or heavy pan, pound to 3/4" thickness.
In a plastic resealable food storage bag, combine the oregano, scallions, vinegar, oil, pepper, and salt. Add the chicken, seal, and turn to coat. Chill for 2 hours.
Lightly coat a grill rack with nonstick cooking spray. Preheat the grill to medium for indirect heat. (If using a charcoal grill, position the coals on one half of the grill. If using a gas grill, heat one side to high, the other to low.)
Remove the chicken from the marinade. Place the chicken on the hottest section of the grill. Cook for 6 minutes, turning once. Move the chicken to the cooler section of the grill and cook for 6 minutes more, turning once, until a thermometer inserted into the thickest part of the breast registers 165°F.