Preheat the oven to 180ºC/350ºF/gas 4. If your chicken has been refrigerated, let it come up to room temperature.
Cut the unpeeled potatoes into 2cm-thick chips, then parboil in a pan of salted water with one of the whole lemons for 4 to 6 minutes.
Drain in a colander (remove the lemon and set aside for the dressing), leave to steam dry for a couple of minutes, then shake them around a little to create rough edges – this will help to make them nice and crispy.
Massage a little olive oil, sea salt and black pepper into the chicken. Halve the remaining lemon, then rub one half over the bird, before popping both halves into the cavity.
Place the chicken, breast-side-down, directly on the grills in the middle of the oven – most of the fat is in the back of the bird, so this will allow the juices to run into the breast and keep all of the meat moist.
Position a tray on the shelf below to catch the juices (you’ll need these for the chips), and roast the chicken for 1 hour 30 minutes, or until cooked through, turning the chicken over after 45 minutes to crisp up the rest of the skin.
Once you have 30 minutes of cooking time left, add the parboiled chips and thyme to the tray with the chicken juices, stirring to coat. Smash and add the garlic, season it all and pop in the oven to cook, tossing after 15 minutes.
Once the chicken is cooked, take it out of the oven and turn up the temperature to 200ºC/400ºF/gas 6. Leave the chips in for 5 to 10 minutes, or until crisped up, while the chicken rests.
To make the dressing, halve the boiled lemon and scoop the jammy insides into a bowl. Very finely chop half of the skin (avoiding the bitter pith) and add this too.
Pick and finely chop the basil and parsley leaves along with the chives. Add to the lemon with the yoghurt. Stir, season and loosen with a little extra virgin olive oil. Set aside.
Reserving the bushy tops, finely slice the fennel, finely shred the cabbage and peel the carrots lengthways into ribbons.
Add the veg to a large serving bowl, toss through the lemony dressing, then scatter over the saved fennel tops. Herb tops and baby leaves are delicious here too.
Serve the roast chicken with the chips and lemony salad on the side.