First, make the pâté, which needs to chill for at least one hour, but will keep in the fridge in a sealed container for up to three days. Break the mackerel fillets into a blender or food processor, double checking carefully for any small bones. Add the cream cheese, crème fraîche and a squeeze of lemon, and blend until smooth. Transfer to a mixing bowl, add the chopped herbs and season with salt and pepper.
Divide the pâté among preserving jars and bang the jars on the counter to dispel any air. Place in the fridge for at least one hour, or until required.
Next make the pickled cucumber. Allow enough time for the brine to cool and the cucumbers to sit in the brine for least two hours at room temperature and then chill for at least an hour before serving, but they will keep in a covered container in the fridge for up to two days. Place the water, sugar, vinegar, bay leaves, thyme sprigs and a pinch of salt in a heavy-based saucepan, stirring to dissolve the sugar. Bring to the boil, then set aside and leave to cool completely.
Once the brine is cool, pour it into a non-metallic bowl. Cut the cucumbers into 0.5cm-thick slices, add to the brine and leave for two hours, then transfer to the fridge if not using immediately.
When you’re ready to serve, remove the cucumber slices from the brine and pat dry. Spread the pâté on pickled cucumber slices and serve with oatcakes on the side, if you like.