Place the chicken on a large cutting board, breast side down and with the tail side facing you. Pat it very dry with paper towels.
Using kitchen shears, begin cutting from the tail end on one side of the backbone all the way to the neck. Repeat the same cut on the other side of the backbone. You can cut away any loose overhanging skin by the neck opening. Save the backbone for chicken stock. Flip the chicken over and with two hands press the breast bone down until the chicken is splayed or flattened. You should hear a couple of cracks during this process. The goal is to flatten the chicken for even cooking.
Preheat oven to 425°F and set the oven rack to middle level.
In a large bowl combine the onion, garlic, rosemary, parsley, oregano, grated cheese, and olive oil. Mix well.
Spread the potatoes out onto a large baking sheet or pan. Pour half of the oil mixture onto the potatoes and toss to coat. Lightly season the potatoes with salt and pepper.
Dry the chicken off one more time, then season both sides with a total of 2 teaspoons of kosher salt and 1 teaspoon of black pepper. Place the spatchcocked chicken breast-side up on top of the potatoes then pour the remaining oil mixture onto the chicken. Use your hands to really get the chicken coated above and below the skin on all sides.
Bake the chicken for 50-70 minutes or until an instant read thermometer reads 158-160°F when inserted into the thickest part of the chicken breast.
If desired you can broil the chicken for a few minutes to crisp up the skin, but watch carefully! Remove the chicken to a platter and sprinkle with the remaining 1 teaspoon of kosher salt. Let the chicken sit lightly tented with foil for 10-15 minutes before serving. The chicken will reabsorb some of the pan juices and become moister.
Flip the potatoes over in the pan and bake at 450°F for 5-10 minutes or broil for a few minutes to crisp them up.
Serve the potatoes and chicken on a large platter. Enjoy!