In a small skillet, saute onion and garlic in 3 tablespoons butter for 2-3 minutes or until tender. Remove from the heat; stir in bread crumbs. Carefully cut a pocket in each chicken breast half. Fill with bread crumb mixture; secure with metal or soaked wooden skewers.
In a small microwave-safe bowl, melt remaining butter; stir in lemon zest. Grill chicken, covered, over medium heat or broil 4 in. from the heat for 6-8 minutes on each side or until a thermometer reads 170°, basting frequently with lemon butter.