Preheat BBQ or grill to medium. Brush chicken with 1 tsp of oil and season with pepper.
Bring potatoes to the boil in a saucepan of salted water and cook for 5 minutes or until just cooked. Drain and add remaining oil and onion and cook over very high heat, shaking gently every minute for 5 minutes. Toss in beans and cook for another 3-4 minutes. Remove from heat and toss with mustard and vinegar.
Grill chicken and lemon slices for 3-4 minutes each side, remove and cover loosely with foil for 5 minutes before serving, laying lemon on top of the chicken. Once potatoes are done, serve!