Melt the butter in a large heavy-based pan and gently fry the onion until transparent, then add the carrots and stock. Season with salt and pepper. Bring to the boil and simmer, uncovered, for about 30 minutes.

Liquidize the soup, then return to the pan and add the chopped sage. Bring to the boil and simmer for another 15 minutes. Serve, garnished with sage sprigs, if using.

Fresh sage is usually available in large supermarkets during the winter months. If you cannot find any, dried sage may be used instead. Soak the leaves first in a tablespoon of warmed dry white wine for about 20 minutes.