Preheat the oven to 220ºC/gas 7. Trim and halve the brussels sprouts, then place on a large baking tray. Add the oil, grate over the lemon zest, then sprinkle with the chilli flakes and a pinch of sea salt and black pepper. Mix with your hands to coat. Roast in the oven for 10 minutes – the sprouts should start to caramelise in places; when that happens, scatter over the Parmesan and roast for a further 15 minutes, until the cheese is crisp and golden brown, and the sprouts tender.