Preheat the oven to 180°C/350°F/gas 4. Rub the chicken all over with olive oil, sea salt and black pepper, then brown in a large casserole pan on a high heat, removing it to a plate once golden all over (roughly 5 minutes). Meanwhile, peel the potatoes and chop into 5cm chunks. Pour 700ml of water into the pan, then add the parsley stalks, garlic bulb (reserving 1 garlic clove for later) and the potatoes. Bring to the boil on the hob for 15 minutes, season lightly, then pop the chicken on top and place in the oven for 1 hour 15 minutes, or until the chicken is golden and cooked through.
Meanwhile, peel and finely chop the reserved garlic clove, very finely chop the parsley leaves, and roughly chop the hazelnuts. Decant into a small serving bowl, stir in 3 tablespoons of extra virgin olive oil and 4 tablespoons of water, and season to perfection. Transfer the chicken and potatoes to a serving platter, spoon over the herby salsa and serve the garlic bulb on the side, for squeezing over.