Method

1. For the aioli: In food processor combine Dijon mustard, lemon juice and egg yolk until completely blended. While motor is running, slowly pour in oil until desired thickness is reached. Transfer aioli to medium mixing bowl and fold in mint and lemon zest. Season with salt and pepper.
2. For the onions: In large sauté pan, place 1 tablespoon of oil over medium heat. Add onions and cook, stirring occasionally about 15 minutes until onions are brown and caramelized. Season with salt and pepper.
3. Form the lamb mince into four equal size patties on a baking sheet lined with waxed paper.
4. Flatten the patties so they are slightly larger than the diameter of the buns.
5. Lightly brush the lamb patties with olive oil and season each with salt and pepper on both sides, then place the patties on the grill.
6. Cook the burgers until fully cooked through and an instant-read thermometer registers 70°C when inserted into the center of the patties, about 4-5 minutes per side.
7. When the burgers are almost finished cooking, brush the cut sides of the buns lightly with olive oil and grill, oiled side down, until lightly toasted with grill marks, about 30 seconds to 1 minute.
8. Remove the buns and burgers from the grill and allow the burgers to rest for 3 minutes.
9. To assemble: Top bottom half of buns with the caramelized onions, then the burgers, and then lettuce, tomato and mint aioli. Finish with the bun tops and serve.