Remove the backbone from the chicken by cutting down either side with chicken shears or a boning knife.
Save the backbone for future use (freeze for use in broth for example)
On a cutting board, press the bird flat, cracking the breast bone and flattening the bird so all the skin is facing up.
Pat the bird dry all over using paper towels, dispose of the towels.
Place the bird skin side down, sprinkle the cut side (with no skin), with 1 tablespoon kosher salt and 1 teaspoon black pepper. Flip the bird skin side up, place skin side up in a 12 or 14” skillet or casserole.
Sprinkle the skin side with the remaining tablespoon of kosher salt and the remaining teaspoon of black pepper, rubbing it all over the skin, under the wings, etc.
Dot the skin side with the butter. Halve the lemons and place cut side down in the roasting pan.
Place in the 425ºF oven and let cook for 20 mins. Remove and set on top of the stove, using the fresh rosemary, brush melted butter and pan drippings all over the chicken, set the rosemary alongside the chicken after you have basted it and return pan to the oven.
Bake for an additional 35 minutes. Remove and test the thickest part of the breast with a meat thermometer. The thickest part of the breast should register at 145ºF.
Let rest for 20 minutes before eating. Serve with the rosemary sprinkled on top, with roasted lemon wedges and with chimichurri.