To make the garlic dressing, place mayonnaise, garlic, vinegar, salt and pepper in a bowl and stir to combine. Set aside.
Heat a barbecue or char-grill pan over medium heat. Brush potato with oil and cook for 5–6 minutes each side or until tender. To serve, arrange potato, onion, mint and rocket on a serving plate and drizzle with garlic dressing. Serves 4.