Cut the peppers in half, remove the stem and seeds and chop roughly. Cut the leeks into 5cm pieces. Heat the olive oil in a large saucepan, add the vegetables and sauté for 5 minutes on a high heat, then reduce the heat, add the rosemary, garlic and stock, cover and simmer for 10 minutes. Remove the lid and increase the heat until the liquid has evaporated. Stir in the balsamic vinegar and sugar, and serve.