1. Preheat oven to 450 degrees, with rack in upper third. Place chicken, breast side down, on a work surface. Starting at thigh end, with kitchen shears, cut along one side of backbone, then the other side. (Discard backbone or freeze it for stock.) Open chicken like a book and flip it over. Press firmly on breastbone (as shown) to flatten. Rub chicken all over with 2 teaspoons oil and season with salt and pepper.

2. Line bottom of a broiler pan with foil. Drizzle foil with 2 teaspoons oil and arrange potatoes in a single layer, turning to coat. Top with onions and season with salt and pepper. Cover vegetables with slotted top of broiler pan and place chicken, breast side up, on top. Roast until chicken is cooked through (a thermometer inserted into thickest part of thigh, avoiding bone, should read 165 degrees), 35 minutes. Let chicken rest 5 minutes before serving.

3. Toss watercress with 2 teaspoons oil and orange juice and season with salt and pepper. Divide potatoes and onions among 4 plates, top with watercress, and serve alongside chicken.