To spatchcock the chicken, place the chicken breast side down on a cutting board and use your fingers to locate the backbone which should run all the way down from neck to tail. Use kitchen scissors or a sharp knife to cut along the right side of the backbone, and then cut all the way along the left side of it so that the backbone can be removed. Now flip the bird over and press down firmly on the breasts to flatten the bird. Rinse the inside of the chicken out and pat the skin dry using a paper towel.
Cut the lemon in half lengthwise and then thinly slice both halves. Cut the onion and leek into thin slices. Place the slices of lemon, onion and leek in a large baking dish or cast iron skillet.
Place the chicken on top of the slices vegetables.
In a bowl whisk together the olive oil, garlic, thyme, rosemary, white wine vinegar, salt and pepper. Brush the chicken all over with the herb oil and once well coated, drizzle any remaining oil over the vegetables. Place in the oven to bake for 30 minutes.
After 30 minutes, remove the chicken from the oven, stir the vegetables to ensure they don't burn and then spoon some of the sauce from the pan over the chicken. Return to the oven to bake for another 10 minutes.
After 10 minutes of baking, check the internal temperature of the chicken using a meat thermometer. The chicken is done cooking when the thermometer reads 155 to 160 degrees Fahrenheit when inserted into the thickest part of the breast. Leave the chicken to rest for 10 minutes before slicing and serving with the onions, leek and sauce spooned overtop.