Melt the butter in a frying pan, add the onion and celery while stirring for approx. 4 minutes.
Leave to slightly cool on a plate. Open the crab tin, drain and shred. Blend with the onion-celery mix from the pan, half of the breadcrumbs (from old bread in the food processor), the crème fraîche, the chives and the egg. Season the mixture with salt and pepper. Roll into approx. 12 small balls. Slightly flatten and dip in remaining breadcrumbs.
Heat a thin layer of oil in a non-stick frying pan. Fry the cakes on both sides until golden brown. Drain on kitchen paper. Make the mayonnaise: in the food processor beat the egg yolk, lemon juice, rind and mustard into a foamy mixture. Add the oil in a thin trickle until it becomes a thick mayonnaise. Briefly pulse the tomato cubes and season with cayenne pepper and salt. Serve with the cakes.
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