For the slow-roasted pork belly, preheat the oven to 200C/400F/Gas 6.
Rub the sage leaves and lemon zest all over the pork. Season with sea salt flakes and freshly ground black pepper.
Place the pork skin-side up on a wire rack that fits over an empty roasting tin, and roast for one hour.
Reduce the oven temperature to 180C/350F/Gas 4 and roast for a further two hours.
For the cider and apple sauce, heat the apples, butter, water and cider in a coveredl saucepan for 4-5 minutes, or until the apples are soft enough to beat to a purée. Stir in soft dark brown sugar to taste. Keep warm.
For the creamy mash, cook the potatoes in a saucepan of boiling salted water for 10-15 minutes, or until tender.
Drain the potatoes, then return to the pan and gently warm to remove any excess moisture. Remove from the heat and mash the potatoes with the butter and cream until smooth.
Season, to taste, with salt and freshly ground black pepper. Set aside and keep warm.
Slice the pork and serve with the mash and apple sauce.