Melt the butter in a large heavy-based pan, add the onion and cook for 1 minute until softened. Add the rice and stir to coat in the butter. Pour in the stock and add the carrot and bouquet garni. Bring to the boil, reduce the heat to low, cover and simmer for about 25 minutes, or until the rice is tender and all the liquid has been absorbed. Remove the bouquet garni.