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1
Halve the cucumber lengthways and run a spoon along each half to remove the seeds entirely, so the bowl of the relish stays bright and the dressing is not thinned by the seeds' water. Slice each half thinly across the grain, then slice the shallots and red chilli to a comparable thinness, so every element softens at the same rate once the dressing arrives. Combine all three in a bowl; the cut surfaces should be clean and dry.
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2
Combine the rice vinegar, sugar, and salt in a small saucepan or bowl and stir until the sugar and salt are fully dissolved; no heat is required if the sugar dissolves readily in the vinegar, though gentle warmth may assist. Now taste the dressing — it must register as immediately, predominantly sweet, with a clean, bright acid note arriving after, not sharp, not muted: this is where the dish lives or dies, because once it coats the vegetables the balance cannot be corrected without washing everything. If the sweetness overwhelms entirely, add vinegar by the teaspoon; if the acid sharpness leads, add sugar and re-dissolve — the ratio is approximately 3 parts sugar to 1 part vinegar by volume, but the palate is the final measure.
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3
Pour the dressing over the cucumber, shallot, and chilli and turn briefly to coat, so the pieces are evenly glossed rather than pooled at the bottom of the bowl. The cucumber slices should glisten and the shallot rings soften very slightly at the edges while retaining their crispness at the centre.
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4
Serve immediately alongside the grilled or fried preparation — satay, gai yang, tod man pla, or equivalent — because extended maceration softens the cucumber and waters the dressing, stripping the relish of the contrast it is there to provide. Scatter fresh coriander leaves over the top if using.