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1
Soak the sliced red onion in cold water for 10 minutes, then drain. This removes the harsh raw bite. The onion is ready when its sharpness on tasting has softened to a clean, mild edge.
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2
Butterfly each shrimp: cut lengthwise through the back, opening the shrimp flat. The cut surface must be fully exposed — this is the only face the brief cure will penetrate. Lay the opened shrimp on a cold plate or tray in a single layer.
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3
Combine the chiles, lime juice, and cold water in a blender. Blend until smooth, then strain through a fine sieve. The strained liquid — the agua de chile — should be vivid green, sharply tart on the nose, and completely fluid with no pulp remaining.
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4
Pour the agua de chile over the butterflied shrimp. This is where the dish lives or dies: the cure window is 2 to 5 minutes only. Watch the shrimp — the edges will just begin to turn opaque while the centre remains translucent and almost raw. Beyond 5 minutes the texture crosses into ceviche; the eating experience is a different dish. Do not refrigerate during the cure; room temperature is required for the flavour to remain vivid.
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5
While the shrimp cure, arrange cucumber rounds across four chilled plates, overlapping slightly. Lay avocado slices and drained onion rounds over the cucumber. The plate should be assembled and ready before the cure is complete.
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6
At the moment the cure is complete, lift the shrimp directly onto the prepared plates and spoon the remaining agua de chile over. Serve immediately — the dish is finished at service temperature. The shrimp should taste oceanic and bright, the chile heat immediate and significant, the lime sharp and clean.