Boil the beetroot whole in salted water for 15 minutes. Drain and refresh in cold water. Peel and cut into 2.5-cm / 1-inch chunks.
Gently sauté the onion, leek and celery in the butter until softened. Add the beetroot, the stock and bay leaf. Bring to the boil, skim, lower the heat and simmer for 45 minutes.
Remove the bay leaf and liquidize. Return to a clean pan, season with g