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Mix the yogurt and mustard together, then spread over the bread and top with the beetroot slices, overlapping them to make an even base.
For the pastrami tartines, cover the beetroot completely with the pastrami slices, then cut each into six fingers and top each one with a sliver of gherkin.
For the mackerel tartines, cut each piece of bread into eight squares. Top with the mackerel and red onion, then snip over some of the dill. Keep them chilled until ready to serve. Will keep in the fridge for up to one day.