Place a chicken breast in a freezer bag and use a rolling pin flatten them to around 2-3 cm, repeat for the other chicken breasts then place them in the balsamic mix and leave to marinate for at least 30 minutes.
Whilst the chicken is marinating, prepare the coleslaw.
You can either roughly shred all the ingredients (aside from mayonnaise, mustard and lemon) or simply add them into a food processor and coarse blitz them. Add them to a bowl with the mayonnaise, mustard and mix, season and then squeeze the lemon, mix again.
Heat your BBQ or grill pan and lightly brush the surface with oil, grill the chicken for 8 minutes, checking regularly.
Build your burger with the lettuce, tomatoes, gherkins and whatever condiment you prefer.