Bring a large pan of water to the boil. Scrub and wash your potatoes, slice them in half and add them to the boiling water. Boil for about 5 minutes until just tender and slightly translucent. Drain them and then let them air dry on paper towels until needed.
In a mini food processor, combine the tarragon and rosemary leaves, the lemon zest, 2 garlic cloves, 1 tbsp of olive oil and plenty of seasoning. Blitz until it’s a rough paste and set aside.
Flip the chicken so it’s breast side down and, using kitchen shears, carefully trim off any excess skin from the neck opening. Remove the backbone by cutting along each side of it from either end. It can be a little tough going, so give it some welly. Flip the chicken over and open it like a book. Press firmly on the breastbone to flatten it out.
Flip the chicken back over and season well, rubbing salt and pepper all over the chicken. Beginning at the neck end of the breast, carefully loosen skin from the flesh of the breast and thighs with your fingers and spread the herb paste underneath the skin.
Preheat the oven to 240°C/gas mark 7. Heat a large cast iron frying pan on the stove until hot and then add 1½ tbsp of olive oil until the bottom of the pan is completely coated. Add the chicken to the pan skinside down. Weigh the chicken down with a very heavy saucepan lid (we used the lid from a casserole dish) and then cook undisturbed in the oven for 20 minutes.
Remove from the oven, flip the bird and add the red onions. Leave the lid off and continue to cook for another 20 minutes or until the chicken is cooked through.
Meanwhile, heat a large frying pan over a medium heat and add the remaining olive oil. Once hot, add the potatoes in a single layer and cook for 10-12 minutes, turning occasionally until the potatoes are crisp and golden on all sides. Use a slotted spoon to transfer the potatoes to kitchen paper, allowing them to drain, and season with sea salt and cracked black pepper.
Once the chicken is cooked, remove it from the oven and transfer it to a large plate along with the red onion, leaving it to rest for 10 minutes. Transfer the pan the chicken was cooked in and all its juices to the stove and add the rosemary stalk, lemon juice and remaining crushed garlic clove. Add 150ml water and any of the juices accumulated from the chicken on the plate and then bring it all to the boil for 3 minutes, scrapping up the bottom of the pan to loosen any bits of chicken. Strain, then serve with the chicken and potatoes.