Preheat the oven to 180°C/350°F/gas mark 4 .
Heat the sugar in a 25 cm ovenproof frying pan over a medium heat. When the sugar is just golden brown in colour add the plums skin side facing up. Continue to cook on low heat for 1-2 minutes, being careful not let the sugar burn.
Place the puff pastry sheet on top of the plums in the frying pan. Tuck in the edges around the plums and place in the oven to bake for 10 mins or until the pastry is golden brown and cooked through.