Combine the chicken with 45ml water, the onion and rosemary in a bowl, season to taste; cover and refrigerate for 1 hour.
Divide into 4 evenly sized burgers and brush liberally with oil.
Heat a chargrill or pan grill until very hot, add the burgers and cook for 4-5 minutes each side or longer if you prefer, keep them warm. Grill the ham slices on each