Preheat the oven to 180°C. Line a shallow oven tray with baking paper.
Mix all the beef ingredients together in a bowl and season. Form into 4 large patties and rub each with a little olive oil.
Heat a large frying pan until hot and brown patties for about 2 minutes on each side.
Transfer to the tray and place in the oven to cook for 8-10 minutes until cooked in the center.
Heat a dash of oil in a heavy-based saucepan over low heat and add the onions, sugar, and salt. Cook gently for about 30 minutes or until the onions are soft, stirring from time to time. Add the thyme and red wine vinegar and cook for a further 5 minutes.
Slice the buns in half horizontally and oven grill cut sides if wished. Top each bun base with mayonnaise, lettuce, cucumber strips, beef patty, tomato, onion and tomato relish. Finish with bun tops that you have spread with mayonnaise.
Recipe from recipes.co.nz