Preheat grill to high.

In medium bowl, stir together brown sugar, garlic powder, onion powder and salt. Rub mixture all over split breast pieces, being sure to get under the skin; discard any unused rub.

Place breasts on grill and cook over medium-high, indirect heat.

While breasts are cooking, drizzle about 1 tablespoon of oil on onions. Grill over direct heat about 5 minutes, or until lightly browned.

In food processor, combine vinegar, mustard and peppercorns; pulse until beginning to blend. Slowly add remaining oil, pulsing to incorporate. Add grilled onions and pulse until mixture is well blended.

Remove breast from grill when a meat thermometer inserted into the thickest part of the breast reaches 170°F, 45 to 55 minutes.

Serve with green onion sauce.