Slice cucumbers into thin rounds or half-moons. Place in a large bowl with sliced red onion.
In a small bowl, whisk together peanut butter, lime juice, soy sauce, maple syrup, sesame oil, and minced garlic until smooth.
Add warm water one tablespoon at a time, whisking until dressing is pourable but still thick.
Pour dressing over cucumbers and onions. Toss gently to coat evenly.
Fold in most of the chopped peanuts and cilantro, reserving some for garnish.
Let salad sit for 5 minutes to meld flavors. Garnish with remaining peanuts and cilantro before serving.