1 small red onion
2 skinless boneless chicken breasts (about 1 pound, 5 ounces)
3 garlic cloves smashed
1 red chile halved, thinly sliced, divided
2 cups cilantro leaves with tender stems divided
1 tablespoon Diamond Crystal or 1¾ teaspoons Morton kosher salt divided, plus more
3 tablespoons vegetable oil
1/3 cup peanuts preferably skin-on
Juice from 1 lime (about 2 tablespoons)
1/2 medium head of green cabbage very thinly sliced
2 medium Persian cucumbers thinly sliced on a bias
Pro move: Save the spicy broth and sprinkle with some chopped cilantro or scallions to sip on later. See two more ways to poach chicken here.
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.