Season the chicken all over with salt and pepper. In a large Dutch oven over medium-high, heat 1 tablespoon of oil until barely smoking. Add the chicken in a single layer and cook on each side, without disturbing, until deep golden brown, 3 to 5 minutes per side.
Add the broth and bring to a boil. Cover, reduce to low and simmer until a skewer inserted into the largest thigh meets no resistance, 10 to 15 minutes.
Transfer the chicken to a serving dish and cover with foil. Bring the cooking liquid to a boil over high and cook until reduced to 1 cup, about 5 minutes. Transfer to a small bowl or measuring cup and set aside.
In the same pot, heat the remaining 1 tablespoon oil over medium until shimmering. Add the garlic, aji amarillo paste, ancho chili and cumin, then cook, scraping the bottom, until browned and fragrant, about 1 minute.
Stir in the onions, bell pepper, 2 teaspoons salt and ¼ teaspoon pepper, then add the vinegar. Cook, stirring, until the onions have begun to soften and the sauce is just thick enough to coat the vegetables, 2 to 3 minutes.
Add the reduced broth, bring to a simmer over medium-high and cook, stirring, until thickened to a glaze, about 5 minutes.
Pour the sauce and vegetables over the chicken. Let rest for 10 minutes, then serve.
