1. Preheat the oven to 425°. In a blender, puree the orange zest and juice, lime zest and juice, garlic, ancho powder, oregano and 1/4 cup of oil. Season with salt and pepper.
2. Arrange the chicken in a shallow baking dish, skin side up, and make 3 deep, crosswise cuts in each breast half. Season the chicken with salt and cover it with the puree, rubbing it into the slits. Spread the chayote, poblano and onion on a rimmed baking sheet, drizzle with oil and season with salt. Roast the chicken in the upper third of the oven for 30 minutes, basting a few times, until just white throughout; rotate the pan once halfway through. At the same time, roast the vegetables for about 30 minutes, until browned and the chayote is crisp-tender.
3. Preheat the broiler. Broil the chicken 6 inches from the heat until well glazed, 2 minutes. Garnish with the herbs and serve the chicken and vegetables with the pan juices.