Step 1

Cut onion in half. Coarsely chop one half, then thinly slice the other half; set slices aside. Transfer chopped onion to a medium pot. Add chicken, garlic, half of chiles, 1 cup cilantro, 2½ tsp. salt, and 4 cups water. Slowly bring to a bare simmer over medium heat. Once liquid begins to simmer, reduce heat to low and cook until juices run clear when thickest part of chicken is pierced, 10–12 minutes.

Step 2

Transfer chicken to a cutting board and let cool; coarsely shred. Reserve ¼ cup poaching liquid and chill until ready to use. (Strain and reserve any remaining poaching liquid for another use.)

Step 3

Heat oil in small skillet over medium. Cook peanuts, stirring often, until golden, 3–5 minutes. Using a slotted spoon, transfer peanuts to cutting board; coarsely chop. Reserve cooking oil.

Step 4

Whisk chilled poaching liquid, reserved peanut oil, and lime juice in a large bowl. Add cabbage, season with salt, then massage with your hands for a minute to soften. Add cucumbers, chicken, remaining chiles, remaining 1 cup cilantro, reserved sliced onion, and half of peanuts; season with salt and toss to combine. Top with remaining peanuts before serving.

Step 5

Do Ahead: Chicken can be poached 2 days ahead. Let chicken and broth cool separately. Wrap chicken and transfer broth to an airtight container; chill.