Peel the cumbers, slice them in half lengthwise, and remove any seeds with a teaspoon. Slice the cucumber into very thin slices crosswise using a sharp knife or a mandolin. Trim the jalapeño, slice lengthwise, remove the seeds, and finely mince.
In a large bowl, combine the jalapeño, shallots, soy sauce, lime juice, and sugar and stir to combine. Add in the cucumber and toss to combine. Transfer the mixture to a serving bowl or platter, top with the cilantro and the peanuts, and serve.
Notes
If you want to bump up the flavor even further, substitute fish sauce for half of the soy sauce. It will add a layer of pungency to the dish, and even more authentic Thai flavor.
This is best made shortly before serving, though leftovers can be refrigerated for a couple of days. The cucumbers will lose some of their crunch, but they will become even more flavorful. If possible, add the peanuts just before serving.