Step 1 In a medium saucepan, melt butter over medium-low heat, add leeks and cook until soft, about 5 minutes. Add cauliflower, milk and salt and bring to a simmer. Cover and simmer on low heat until vegetables are soft, about 25 minutes. Do not boil.
Step 2 Remove the vegetables from the heat, let cool slightly and puree them in a blender, with an immersion blender or in a food mill. (If using an immersion blender, cover with a towel to avoid splattering.)
Step 3If serving warm, reheat gently and serve with a drizzle of walnut oil. If serving cold, chill in the refrigerator before serving (also with walnut oil).