Preheat the oven to 200°C, fan 180°C, gas 6. Put the chicken, breast-side down, on a large chopping board. Using a large, heavy knife or sturdy kitchen scissors, cut down either side of the backbone of the chicken. Remove the bone and open out the chicken and press to flatten with the heel of your hand.
Using a pestle and mortar or a mini processor, mash or blitz the garlic, lemon zest and thyme leaves to a coarse paste, stirring in the olive oil to loosen.
With a sharp knife make slashes over the flesh of the chicken and rub the mixture in with your hands, pushing it into the slashes all over the chicken.
Spread the potato slices over the bottom of a large roasting tin or shallow baking tray and rest the chicken on top. Pour the chicken stock into the bottom of the tin and roast for 45 mins until golden and the juices run clear from the chicken when pierced with a knife.
Carve the chicken into pieces and serve with the potatoes with a good grinding of black pepper and the pan juices spooned over. For toddlers, chop the chicken (without the skin) into pieces and serve with the potatoes and a little of the pan juices spooned over the top.
Cook's tip: For babies, put some of the chicken (without the skin) in a blender with some of the pan juices and blitz to a loose coarse purée. Add a little potato and some mashed veg, such as carrot or butternut squash.