Place the chicken breast-side up. Cut through the middle of the breast along the breastbone, without separating it into halves. Open like a book and press flat.
Mix the salt, pepper, Dijon mustard, and garlic.
Rub the marinade inside and out. Loosen the skin gently and push garlic underneath.
Cover and refrigerate overnight, or marinate at least 1 hour at room temperature. Let chilled chicken sit out 30 minutes before roasting.
Cut the potatoes, zucchini, leek, and peppers. Toss on the tray with the olive oil and salt.
Place the flattened chicken on top of or in the middle of the vegetables.
Put the tray in a cold oven and set it to 375°F (190°C).
Stir the vegetables every 20 to 25 minutes.
For extra crispness, switch on the convection fan. Remove when golden and crisp.
Check that the juices run clear and the thickest part of the breast reads 165°F (74°C). Rest 10 minutes before carving.
