Place the chicken with the breasts facing up. Using sharp kitchen shears, cut along one side of the backbone, from end to end.
Cut along the other side of the backbone, completely removing about about 1-inch wide portion of the spine.
Turn the chicken over and use your hands to press down firmly on the bird, until you hear the breast bone crack. This will flatten the bird. Optionally, you can tuck the wings.
Marinate
Lay the spatchcocked chicken on a parchment or foil lined sheet pan. Pat it dry with a few paper towels, then rub it all over with 2 tablespoons of oil and the spices, using your fingers to rub it under the skin too.
Let the chicken marinate in the seasonings for at least 30 minutes.
Prepare the Dish
Preheat the oven to 450°F (230°C).
Meanwhile, wash the vegetables. Trim and remove any large stems from the Brussels sprouts, then cut each sprout in half. Cut the potatoes into quarters. Cut the lemon into quarters.
Transfer the vegetables to a bowl, and toss with the remaining oil; season them lightly with salt and pepper.
Spread the vegetables and lemon pieces into an even layer on the sheet pan.
Roast
Set the sheet pan in the preheated oven and roast for 40 minutes, using a meat thermometer to ensure the internal temperature has reached 163℉ at the thickest part of the thigh (*see notes, below).
Allow the chicken to rest for 5-10 minutes before slicing.
Video
Notes
Chicken is safe to eat when the internal temperature reaches 165°F. at the thickest portion of the thigh. To ensure juicy meat, remove the roasted chicken when the internal temperature is 163°F. As the meat rests for 5 minutes, the internal temperature will increase another 2-3 degrees.When taking the internal temperature, be sure the probe isn't touching any bones.To store leftovers: Transfer to an airtight container and keep in the refrigerator for 3-4 days. Alternately, you can freeze leftover chicken for use within 4 months, per USDA.