Fresh Herring Fillets with a Lemon and Mustard Cream

I recall that as a child, when I lived in Wiveliscombe in Somerset, the fishmonger bought his van round every Friday. So, Friday lunch-time we always had deep-fried cod and chips, and for Saturday tea either baked, soused herrings (that is, cooked with onions and malt vinegar) or fried herrings with Granny’s mustard sauce. This is my version.