Preheat the oven to 200ºC (400ºF/gas mark 6). Place the onion, leek, garlic and potatoes in a roasting tin and drizzle over half the olive oil. Toss to coat and then nestle in the chicken thighs. Season with salt and pepper and pour around the chicken stock, then roast for 30 minutes until the potatoes are tender.
Meanwhile, put the kale into a bowl with the rest of the oil and a pinch of salt, then using your hands massage the oil into the kale.
Remove the tin from the oven and stir in the kale, and sprinkle over the vinegar. Return to the oven for 10 minutes until the chicken is cooked through and golden brown and the kale has started to crisp up. Scatter over the thyme and serve straight to the table family-style.