Caramelise the sugar in a frying pan. Halve the bananas lengthways, leaving the skin on. Turn the bananas in the caramel, only frying the sliced side. If the caramel starts to burn, add a little water. Add the finely chopped rosemary and stir through the butter. Then place the bananas onto the lightly salted mascarpone and drizzle with the caramel.