Step 1

For the aioli, combine mustard and egg yolks in a food processor until well blended. With the motor running, slowly pour in oil, until mixture has thickened and emulsified. Transfer to a medium bowl and fold in zest, juice and mint. Season to taste.

Step 2

To make the caramelised onions, heat oil in a large, heavy-based frying pan over low-medium heat. Add onion and cook, stirring occasionally, for 15 mins, or until golden brown and caramelised. Season to taste.

Step 3

To make the lamb burgers, form mince into four equal-sized patties on a baking tray lined with baking paper. Flatten them so they are slightly larger than the diameter of the buns. Preheat barbecue to high.

Step 4

Lightly brush burger patties with oil and season with salt and pepper on both sides. Remove from baking tray and cook for 4-5 mins on each side, or until cooked through.

Step 5

When burgers are almost cooked, lightly brush the cut sides of the buns with olive oil and cook on the grill, oiled side down, until lightly toasted with grill marks.

Step 6

Remove buns and burgers from grill and allow burgers to rest for 3 mins.

Step 7

To assemble, top bottom half of buns with caramelised onions, burger, lettuce, tomato and aioli. Finish with bun tops and serve.