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Begin by preparing the onion purée. Peel and slice the onions, then add to a pan with the butter and thyme and cook slowly on a very low heat until caramelised - this should take about 4 hours. Add the double cream to the pan with the caramelised onions and bring to the boil, then remove from the heat and transfer to a blender, discarding the thyme. Add the milk and salt to the blender and blitz until smooth. Pass the purée through a fine sieve and refrigerate until required