Place the onions in a small bowl of cold water. Let sit for ten minutes, then drain well and pat dry with a paper towel.
Make the dressing: In a large bowl, whisk the lime juice, oil, fish sauce, sugar, and garlic.
Add the drained red onions, cucumbers, cilantro, peanuts and jalapeño pepper to the dressing. Taste and add the salt if necessary. Serve the salad within 30 minutes.
