Put the mince, onion, breadcrumbs, apples, herbs and some seasoning in a bowl. Mix together with your hands. Wet your hands, then divide the mixture into four portions and shape each into a burger. Chill for 1 hour to firm up.
Brush the chicken burgers with a little oil and cook over a medium heat in a non-stick frying pan until browned on both sides, then reduce the heat a little and continue cooking until the burgers are completely cooked through – this should take about 20 minutes in total. Turn from time to time to ensure cooking is even.
Meanwhile, mix together the honey and mustard. Spread the mixture over the top cut surface of each bun. Divide the watercress among the buns, piling it on the bottom cut side.
When the chicken burgers have finished cooking, transfer them to the buns, placing them on the watercress. Put on the tops and serve.