Preheat oven to 450ºF. Place the chicken, breast-side down, on a cutting board. Make sure the chicken is clean and dried thoroughly.
Using kitchen shears, cut along both sides of the backbone and remove it.
Optional: Place the backbone in a plastic bag or plastic wrap, then store in the freezer for making chicken stock.
Flip the chicken over, breast-side up.
Place the palm of your hand over the breast and press down firmly until you feel the breastbone crack. The chicken should lie relatively flat now.
Transfer the chicken, breast-side up, to a wire rack or wire cooling rack. Set the wire rack on a baking sheet covered with parchment paper. Tuck the wings under the body and rotate the thighs to lie flat. In a small bowl, whisk together extra virgin olive oil, Dijon mustard, and lemon juice.
Before you brush the mixture over the entire chicken make sure the chicken skin is dry. The drier the skin is before you apply the seasoning, the crispier the skin will be after it is roasted.
Brush olive oil and seasoning mixture over entire chicken.
Season generously with salt and pepper.
If desired, add 1" pieces of preferred veggies on the parchment paper-lined pan under the rack of chicken.
Roast chicken in the oven for 45 minutes or until cooked through. When a thermometer is inserted into the thigh to check for doneness it should read 165ºF or higher. After the chicken is roasted, transfer to a cutting board and let rest for 10 minutes. If desired, garnish with chopped sage or parsley.