Preheat broiler. Wash chicken and split down the middle into halves. Combine butter, oil, mustard, cayenne and lemon juice in a small mixing bowl. Brush chicken with ½ mixture. Next, add onions, garlic and salt into remaining mixture. Place chicken in a broiler pan, skin side down. Broil for 20 minutes, then turn and broil other side for 10 mintues. Baste with remaining butter mixture and broil another 10 minutes until chicken is tender. Serve with lemon wedges.