Heat oil and butter in a large saucepan over medium-high heat, add leek and garlic, stir occasionally until soft (4-5 minutes). Finely chop cauliflower and potato in a food processor, add to pan, cook for 5 minutes. Add stock, milk and herbs, season to taste, bring to the boil, reduce heat to low and simmer until vegetables are tender (15-20 minutes). Discard herbs, add crème fraîche and lemon rind, process with a hand-held blender until smooth, stir in cheddar and keep warm.
Meanwhile, for thyme butter, heat oil and butter in a small saucepan over medium-high heat until butter foams, add shallot, cook until golden (5 minutes), add thyme, cook until fragrant (1 minute), add lemon juice and season to taste. Served drizzled over soup.
This recipe is from the August 2012 issue of Australian Gourmet Traveller.