Turn your oven to 250C (480F).

If using the whole chicken, place on a chopping board breast side down. Cut down each side of the back bone using a sharp knife or poultry shears. You'll need to cut through the bones so be prepared to use your muscles and your sharpest knife. Remove the backbone and discard (I freeze it to make stock). Then turn the chicken over so the breast is facing upwards. Use your hands to press onto the middle of the breast to flatten out the bird. Then cut lengthwise down the middle of the breast to create two even halves. You'll need to cut through some bone but it won't be as difficult as the back. Then lift up one leg and cut between the thigh and breast to separate them. You shouldn't need to cut through bone this time. Repeat with the other half so you end up with 2 breasts and 2 legs. Give yourself a pat on the back!

Line a large rimmed baking sheet with baking paper (for easier clean up). Place the chicken pieces on the tray. Slice 2 lemons into rounds about 5mm (1/4 inch) and place on the tray. Save the other lemon for making the sauce. Cut each onion lengthwise into 8 wedges, discarding the skin. Place them on the tray as well.

In a small bowl mix 2 tablespoons of the za'atar (save the rest for the sauce), all spice, salt and 2 tablespoons olive oil. Drizzle the spiced oil over the chicken, smearing it with the back of a spoon to coat as much of the chicken as you can.

Pop in the oven on the middle shelf. Set your timer for 20 minutes.

While the chicken is roasting, make the sauce by mixing the last tablespoon Za'atar with 2 tablespoons lemon juice, 2 tablespoons extra virgin olive oil and a pinch of salt. Taste and season with more salt or lemon as needed. Make a mental note that this sauce is divine on all sorts of things especially eggs.

When the timer rings for the chicken, carefully remove it from the oven. Test it with a meat thermometer by inserting the thermometer into the thickest part. I generally test a few places to get a feel for how we're travelling. If it reads 73C (163F) or higher it's done. But you'll probably need to turn the chicken over so the skin side is touching the tray. And scoop the onion and lemon into the middle so they don't burn (or remove the onion and lemon and keep in a clean bowl in a warm place).

If the chicken needs longer return to the oven for another 5-10 minutes. Test again as above.

When the chicken is 73C (163F) or higher. Divide the chicken between 4 plates. Drizzle over the sauce and serve with salad leaves on the side. If you can, wait 5-10 minutes for the chicken to rest before tucking in - this will help the chicken stay moister.