Drizzle the cut sides of the buns and tomatoes with a little olive oil, sprinkle with sea salt, then set aside.
Mix all the ingredients for the harissa tahini to a smooth paste and set aside.
Mix the sliced onions with the rest of the pickled onion ingredients and set aside.
Mix all the burger ingredients until well combined, then put a little oil on your palms, divide the mix evenly and shape into balls weighing about 90g each.
Put each chicken burger on a 12cm square sheet of baking paper and drizzle a little olive oil on top.
Fire up the barbecue and set a heavy cast-iron pan on it; if you’re cooking this indoors, set a heavy-based cast-iron pan and a griddle pan on the hob.
Put the buns and tomatoes cut-side down on the griddle, until they take on a nice char pattern and a little smoke, then transfer to a plate.
One at a time, lay a burger meat-side down on the griddle, and press down on the paper with a flat-bottomed heavy pan to flatten it out to just over the size of the bun.
Cook for three minutes, then flip and cook on the other side for two minutes more.
Repeat with the remaining burgers.
Spread both cut sides of each bun with harissa tahini, then lay a lettuce leaf or two on the bottom bun.
Top with a burger, slices of the grilled tomato and some pickled onions Pop on the lids and serve.
