Heat the oil in a saucepan and add the onions. Cook, stirring continuously, for about 10 minutes or until they turn a dark golden brown. Be careful not to let the onions burn. Add two of the garlic cloves, the remaining vegetables, herbs and yeast extract. Cook over a high heat for 4-5 minutes or until the vegetables are lightly browned.

Add the water to the pan and bring to the boil. Lower the heat and simmer gently for 30 minutes. Strain through a fine sieve into a jug or bowl. The stock is now ready to use, or it can be returned to a clean saucepan and boiled rapidly to intensify the flavour. Cool and store in the refrigerator for 2-3 days or freeze for later use.

Drain the chestnuts and put them in a saucepan. Cover with cold water, bring to the boil and simmer for 20-30 minutes or until tender, then drain.

Put the remaining garlic and chestnuts on a baking tray and roast in the oven at 200 �C / 400 �F / Gas 6 for 20 minutes or until the garlic is soft to the touch and the chestnuts are lightly browned. Cool slightly, then pop the garlic cloves out of their skins.

Put the garlic, chestnuts and stock in batches in a blender or food processor and process until smooth. Pour into a saucepan, add the sherry and season with nutmeg. Bring to the boil, then reduce the heat and simmer for 10-15 minutes.

Serve the soup in warmed bowls and top each bowlful with a tablespoon of double cream.