To make the lamb patties, combine ingredients in a bowl, season to taste and mix well with your hands to combine and until the mixture starts to hold together. Divide into four and shape each into a flat patty that’s slightly larger than your bun, place onto a tray and refrigerate for 30 minutes.
While the patties are chilling, bring vinegar, sugar, 1 ½ tsp salt and 200ml water to the boil in a saucepan, remove from heat, and onion and set aside to pickle. Drain before using.
Mix mayonnaise, cumin and lemon juice in a bowl to combine and season to taste.
Heat oil in a large frying pan over medium heat, add lamb patties in batches and fry, turning occasionally until browned and still a little pink in the centre (5-6 minutes), set aside to rest for a minute.
To serve, toast cut – sides of buns and spread bases with cumin mayonnaise. Top each with a patty, feta, pickled onion to taste and lettuce. Sandwich with tops and serve.
If the lamb mince begins to stick to your hands while forming the patties, try wetting your hands with a small amount of water.
Try using lamb leg steaks, or lamb backstrap (eye of loin) as an alternative to mince.