Heat oven to 450 degrees. Place scallions in a single layer on a rimmed sheet pan and drizzle with oil and season with salt and pepper. In a medium bowl, season potatoes with salt, pepper, and oil. Spread on top of the scallions. Season chicken with oil, salt and, pepper and place on top and around potatoes.
Roast chicken until cooked through and potatoes are trendy, 30 to 35 minutes.
Transfer the chicken, potatoes, and scallions to serving plates. Scrape remaining pan juices and drippings into a small bowl, stir in capers and lemon juice, and season with salt and pepper. Spoon over chicken and serve with lemon wedges.