How to Make It
Heat oven to 425°F. On a large rimmed baking sheet, toss the potatoes and onions with 1 tablespoon of the oil, ½ teaspoon salt, and ¼ teaspoon pepper. Dividing evenly, rub the chickens with 1 tablespoon of the remaining oil; season with 1 teaspoon salt and 1/4 teaspoon pepper. Tie the chicken legs together and nestle the chickens in the potatoes and onions.
On a second large rimmed baking sheet, toss the celery and carrots with the thyme, the remaining 2 tablespoons of oil, ½ teaspoon salt, and ¼ teaspoon pepper.
Roast the chickens and vegetables, tossing the vegetables halfway through, until the chickens reach an internal temperature of 165° F in the thickest part of a thigh, 1 to 1¼ hours. Transfer the chickens to a cutting board and let rest for at least 10 minutes. Combine the vegetables on one of the pans and toss with the parsley.
In a small bowl, mix together the mustard and maple syrup.
Reserve one chicken and half the vegetables and sauce for tomorrow’s dinner. Serve the remaining chicken and vegetables with the remaining sauce.
Reheat and serve: Cut the chicken into quarters. Reheat with the vegetables in a covered baking dish at 350° F for 12 to 15 minutes.