Put the cucumber on a board, then use a rolling pin of the flat of a large knife or cleaver to smack it until it breaks open and is partially and evenly crushed.
Chop into rough shapes, then put in a colander set over a bowl. Sprinkle on a generous pinch of sea salt, toss, then put in the fridge for at least 30 minutes to crisp up.
Meanwhile, make the dressing.
Put a pinch each of sea salt, sugar and optional red chilli flakes to taste in a medium bowl, add the vinegar and garlic, then add the oil and soy sauce and mix to combine.
When you’re ready to serve, toss the steeped cucumber in the dressing, until well coated – keep the cucumber juice to drink or, if there’s only a small amount, even add it to the dressing.
Serve topped with an optional sprinkling of sesame seeds and the coriander sprig chopped from stem to leaf.