Step 1

Preheat the oven to 425F. Place the chicken on a cutting board, breast-side-down. Use a sharp pair of kitchen scissors to cut each side of the spine to remove it.

Step 2

Flip the chicken over. Place your hands over the breastbone and push down firmly to break it. Flip the chicken back over and generously brush the inside and back of the legs with about 3 tbsp of the dijon mustard.

Step 3

Arrange the sliced onions on a parchment paper-lined baking sheet, as a bed for the chicken. Place the chicken over the onions. Brush the front of the chicken generously with the remaining dijon mustard.

Step 4

Transfer the pan to the oven and roast for 45 - 50 minutes, until the chicken is completely cooked through and the skin is deeply browned. Let the chicken rest for 5-10 minutes before slicing.

Step 5

Optional: After removing the chicken from the pan, toss the onions well with the drippings and browning on the pan before serving with the chicken.