Remove the top shelf from the oven, then preheat to 200°C/400°F/gas 6. Scrub the potatoes, chop into erratic 2 to 5cm chunks and place in a large baking dish. Peel and quarter the onions, then break apart into petals and add to the dish. Strip half the oregano leaves into a pestle and mortar and pound to a paste with a pinch of sea salt and black pepper, then muddle in 2 tablespoons each of olive oil and red wine vinegar. Drizzle most of the marinade into the dish, toss together well and roast for 10 minutes.
Meanwhile, use a speed-peeler to strip the peel from the lemons, then slice the lemons in half. Toss the chicken legs in the reserved marinade. When the time’s up, arrange the chicken directly on the bars of the top shelf and return it to the oven, so the delicious juices rain down on to the spuds below. Throw the lemon halves into the baking dish, then roast for 1 hour, or until everything is golden, shaking up the potatoes and onions halfway and tossing through the remaining oregano leaves and lemon strips for the last 15 minutes. Carefully remove the chicken from the oven, along with the veg, and squeeze over the jammy roasted lemon halves, to serve.