1 3 1/2–4-pound chicken, backbone removed
1 tablespoon kosher salt plus more
1/2 teaspoon freshly ground black pepper plus more for seasoning
1/4 teaspoon cayenne pepper
4 tablespoons olive oil, divided
2 tablespoons finely grated lemon zest
3 tablespoons fresh rosemary leaves
1 large fresh poblano chile, quartered, seeded
1 large red bell pepper, quartered, seeded
1 red onion, cut into 1/2" slices
1/4 cup (or more) red wine vinegar
2 garlic cloves, sliced
1 cup fresh basil leaves
4 avocados, halved and pitted
1/4 teaspoon chili powder
Ingredient info: Poblano chiles are available at Latin markets, specialty foods stores, and some supermarkets.
Special equipment: One brick, wrapped in foil, or a cast-iron skillet