Lemon Chicken with Potatoes and Escarole

Preheat oven to 450°F, with a rack in upper third. Place potatoes in a pot; cover with 2 inches generously salted water. Bring to a boil and cook until almost tender, 3 to 5 minutes; drain. Remove zest from lemon in long strips with a peeler, then use a sharp knife to slice strips into a fine julienne. Squeeze 3 tablespoons lemon juice into a bowl, whisk in oil and capers, and season with salt and pepper.

Season both sides of chicken with salt and pepper; arrange in an even layer on a rimmed baking sheet with onion and potatoes. Pour caper mixture over everything; toss to coat. Roast until chicken is golden brown and cooked through, 45 to 50 minutes; transfer to a plate.

Switch oven to broil and cook vegetables, stirring once, until juices have reduced slightly and vegetables are charred in places, about 5 minutes. Stir in julienned lemon zest and escarole; broil another 30 seconds. Serve with chicken and mustard.