Heat the grill to medium-hot. Boil the potatoes for 20-25 mins or until tender.
Meanwhile, use your hands to squeeze any excess juice from the apple, then place in a bowl with the mince, onions, breadcrumbs, mustard, parsley and some seasoning. Use your hands to mix the ingredients until well combined. Add half the egg (reserve the rest for Chinese leaf pork wraps, see 'goes well with') and mix again. With clean, wet hands (to prevent sticking), shape the mixture into 2 burgers. Grill the burgers on a foil-lined tray for 18-20 mins, turning once until golden.
Drain the potatoes, toss in the tzatziki straight away and serve with the burgers and rocket, if you like.