Heat the oil and butter in large saucepan, add the leek and the garlic, cook over a medium heat for 6-8 minutes, or until the leek is soft but not brown. Increase the heat to high, add the cauliflower, potatoes and stock, season with salt and pepper and bring to the boil. Reduce heat and simmer for 20 minutes, or until the cauliflower and potato are tender.
Puree until smooth. Return to saucepan and add the cream and lemon juice.
Reheat gently for 5 minutes.
Serve topped with chopped chives.

Serves 6.