Place the lamb mince in a large bowl with the onion, breadcrumbs, mint, cumin, feta and a generous pinch of salt and pepper. Using your hands mix until fully combined then divide the mixture into 8 portions. Roll each portion into a ball and flatten slightly. Place on a tray, cover and refrigerate for 15 minutes.
Heat a large frying pan or grill over medium heat, brushing with a little oil. Cook the rissoles (in batches if necessary) for 3-4 minutes each side until cooked through. Remove and drain on paper towel.
Serve rissoles with tzatziki, warm pita bread and a Greek-style salad.