Preheat the oven to 200°C, fan 180°C, gas 6. Toss the potatoes in the oil. Cook on a non-stick baking tray for 30 minutes, turning a few times.
Meanwhile, fry the onion in a non-stick pan over a medium heat for 10 minutes, adding the garlic for the last minute. Allow to cool.
Mix the mince with the onion, egg, mixed herbs and breadcrumbs. Shape into 4 burgers, then chill in the fridge for 15 minutes.
Preheat the grill to medium-high. Grill the burgers for 5 minutes on each side, or until cooked through completely. Serve in baps spread with mayo, lettuce leaves, avocado and tomatoes.
** Wash hands thoroughly after handling raw meat**