For the pastry cases, preheat the oven to 180C/350F/Gas 4.
Lay one sheet of filo on a clean work surface and brush with melted clarified butter. Lay another sheet of filo on top (ensuring there are no creases) and brush with more clarified butter. Repeat until you have four layers of filo pastry.
Cut out circles using a 10cm/4in pastry cutter. Gently press the circles into the tartlet moulds (or bun tin). Add a layer of heat proof cling film or parchment paper over the pastry and fill with baking beans. Cook for 15 minutes, or until light golden-brown, to blind bake.
For the filling, in a food processor combine the oysters, egg yolk, Dijon mustard, salt, pepper, a squeeze of lime juice and vegetable oil. Blend to the consistency of mayonnaise (if needed, add some juice from the oysters to loosen the mixture). Transfer to the fridge to chill until ready to use.
Dice the mackerel fillets and put in a bowl. Add the chopped shallots, capers, ginger, soy sauce, a squeeze of lime juice and 1-2 tablespoons of the oyster mayonnaise. Add the chopped chives and season with salt and freshly ground black pepper.
For the pickled cucumber, in a pan bring the vinegar, sugar, chilli and ginger to a boil. Use a vegetable peeler to slice the cucumber into 7cm/3in long strips. Cool the vinegar mixture, then add to the cucumber.
To serve, fill the tartlets with the oyster mayonnaise and top with the pickled cucumber and some samphire if you like.