Combine the yogurt with the mayonnaise and grated garlic in a small bowl. Heat the vegetable oil in a non-stick frying pan over a medium heat, add the lamb burgers and fry for around 10 mins, turning over halfway through, until piping hot in the centre. Lightly toast the burger buns. Divide the sliced tomato, sliced gherkins, the burgers, some garlic sauce, chilli and the lettuce leaves between each burger bun, then enjoy immediately.