30 minutes before cooking, remove the joint from the fridge. Preheat the oven to 200°C, fan 180°C, gas 6. Rub the beef with a mixture of cumin, nutmeg, and spice.
Place the onion and lemon wedges in the bottom of a roasting pan, top with the beef, and roast for 1 hour and 15 minutes for medium steak.
Meanwhile, combine the pomegranate juice and sugar in a saucepan. Cook, stirring periodically, for 20-25 minutes, or until the sugar has dissolved and the mixture has reduced by half. Stir in the cornflour and water mixture, then bring to a rapid boil, stirring constantly, for 3-5 minutes, or until thickened and glossy.
Brush the pomegranate glaze generously over the steak after it has been cooking for 40 minutes. 15 minutes before the conclusion of the cooking period, repeat the process. Before serving, take the meat out of the oven and let it rest for 20 minutes. Brush with some of the leftover glaze and place on a serving plate. If using, garnish with pomegranate seeds and mint sprigs. Drizzle the leftover glaze over the beef before serving.