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For the marinade, whisk the first five ingredients together, season with black pepper and a large pinch or two of sugar. Finely grate the zest of half a lemon and set aside, then squeeze the lemon juice from that half into the marinade.
Put the chicken in a nonmetallic dish and spoon the marinade over. Halve the other lemon and add the three halves to the chicken, and marinate until ready to cook. (This can be done up to two hours ahead or overnight if that suits you better.)
Just over an hour before you want to eat, preheat the oven to 200C/gas 6/fan 180C. Cut a thin slice off the top of the garlic (this makes it easier to squeeze it out later) and halve the potatoes or cut them into largish chunks, depending on their size. Drain most of the marinade into a roasting tin and add the potatoes and garlic, tossing to make sure that they are coated in the oil. Roast, uncovered, for about 20 minutes.
Add the chicken, rosemary and lemon to the tin, scraping in any bits of marinade. Toss to mix, arrange the chicken skin-side up and season well with salt, then roast for another 40-45 minutes or until the potatoes are fully cooked, and the chicken is golden and crisp and the garlic is squashy. Mix the reserved lemon zest and parsley, scatter over the chicken and serve immediately with seasonal green vegetables or salad. The best way to eat the garlic is to squash it out of its skin with a knife and spread it onto the roasted potatoes and chicken.