Spicy Chicken and Cabbage Salad

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.

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.)

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.

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.

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.