Lemon-scented carrots and radishes are a winning combination. When I worked in France, radishes were often cooked to accompany meat dishes but in the UK we almost always serve them raw, in salads or as crudités. For this dish, try to find small, young radishes, as large ones can be quite hot.
Place the carrots and radishes in a saucepan, add the butter, lemon juice and zest, then sprinkle over the lemongrass and drizzle over the syrup. Add just enough water to cover. Place a circle of buttered greaseproof paper on top and bring to the boil. Reduce the heat slightly and cook until the carrots are tender and the cooking juices form a glaze around the vegetables. Toss well, then season and serve.