Blend breadcrumbs, garlic, coriander, mint sauce, and onion in a food processor until just combined.
Mix the processed mixture with lamb mince and shape into 8 burgers with wet hands.
Chill the shaped burgers on a tray in the fridge.
Fry onion and garlic in oil for 5 to 7 minutes until soft.
Add remaining relish ingredients, bring to a boil, and cook for 2 to 3 minutes.
Simmer the relish covered for 10 minutes and season with salt and pepper.
Combine mayonnaise, chopped mint, and mint sauce in a bowl.
Heat oil in a pan or light a barbecue.
Cook burgers for 3 to 4 minutes per side until browned.
Toast buns and assemble with relish, lettuce, burgers, mint mayo, and gherkins.