Hint: the mayonnaise can also be made in a food processor. Add the oil in a thin stream, with the motor constantly running, until the mixture thickens and turns creamy.
1 kg (2 lb 4 oz) minced (ground) chicken
1 small onion, finely chopped
2 teaspoons lemon zest
2 tablespoons sour cream
80 g (2¾ oz/1 cup) fresh breadcrumbs
6 onion bread rolls
1 handful rocket (arugula) leaves, to serve
3 tomatoes, cut into wedges, to serve
1 egg yolk
1 tablespoon tarragon vinegar
½ teaspoon dijon mustard
250 ml (9 fl oz/1 cup) olive oil
1. Place the chicken in a mixing bowl. Add the onion, lemon zest, sour cream and breadcrumbs. Using your hands, mix until thoroughly combined. Divide the mixture into 6 equal portions and shape into 1.5 cm (⅝ inch) thick patties.
2. Preheat a barbecue grill or flat plate to high. Lightly oil the hot barbecue. Cook the patties for 7 minutes each side, turning once. Serve the patties on the onion rolls with the mayonnaise.
3. To make the tarragon mayonnaise, place the egg yolk, half the vinegar and the mustard in a small mixing bowl. Whisk together for about 1 minute, or until light and creamy. Add the oil about 1 teaspoon at a time, whisking constantly until the mixture thickens. Increase the flow of oil to a thin stream and continue whisking until all of the oil has been incorporated. Stir in the remaining vinegar and season to taste.
4. Put the patties on the rolls with rocket and mayonnaise. Serve with some tomato wedges on the side and sprinkle with freshly ground black pepper.