Step 1Place the onions in a small bowl and pour the vinegar over them. Turn to evenly coat. In another small bowl, sprinkle ½ teaspoon coarse salt over the cucumber and toss well. Let both stand at room temperature while preparing the sauce and shrimp.
Step 2Arrange the shrimp on a serving platter or individual plates (not a bowl) in a single layer. Sprinkle 1 teaspoon salt over the shrimp. Refrigerate until ready to serve.
Step 3Combine the lime juice, parsley, cilantro, a pinch of coarse salt and ¼ cup water in a blender and puree on high speed until liquified. If it’s frothy (which it probably will be), let it sit for about 5 minutes to settle.
Step 4Spoon the lime sauce over the shrimp, then scatter chile on top. Drain the onion and arrange over the shrimp, along with the cucumber slices. Drizzle with the oil and sprinkle with the finishing salt. Serve immediately.