Pour sugar onto a plate. Cut unpeeled pears in half lengthways and dip generously into the sugar.
Place an ovenproof frying pan over a medium heat. Add butter to pan to melt. Once butter is melted add pears cut side down and rosemary sprigs.Cook over gentle heat until the pears are deeply caramelised. This should take around 8-10 minutes depending on the heat.
Turn pears over. Add the remaining ingredients and stir to combine. Roast pears in the preheat oven until tender about 15-20 minutes. Sprinkle toasted walnuts over to serve.