preheat the oven to 200C/Gas6
to make the bread cases, cut 36 rounds our of the slices using a 5cm cutter then gently flatten each round using a rolling pin
melt the butter with the mustard and brush each round of bread
press buttered side down into miniature muffin tins and brush again with butter and bake for about 10 minutes until golden brown; cool
place the mackerel fillets and lemon juice in a food processor and whiz for a few minutes until smooth
whip the cream until it holds soft peaks and add the mackerel, horseradish, rucola and pepper and fold together
spoon the mixture into the prepared bread cases and top with lemon rind
36 servings