Adjust your oven rack so that it’s in the middle of the oven. Heat oven to 450°F. Foil line a large deep baking tray. Place the onion, orange, lemon, celery, and rosemary sprigs on the foil, then place a wire rack on top of the vegetables.
Pat the turkey dry with paper towels. Flip so that it’s breast side down. Hold the turkey firmly and use a pair of kitchen shears to cut alongside the backbone, starting where the tail meets the thigh. Cut all the way up until the turkey is split up to the neck. Push the turkey open slightly and then repeat on the other side, carefully cutting alongside the other side of the backbone. This side may be a bit trickier, so go slow.
Trim off any excess fat or skin you might see. I took off the tail and the hood of fat near the neck.
Flip the turkey over so that the breast is facing up and push down on the ridge breast bone, hard. You should hear a couple of cracks and the turkey should be flatter.
Place the turkey on your prepared rack and baking sheet. Tuck the wing tips behind the breast and rub with 1 tablespoon of oil. Season with salt and freshly ground pepper. Transfer to the oven and roast, for about an hour and twenty minutes, rotating halfway, or until the deepest part of the breast is 150°F, and the thighs are at least 165°F.
While the turkey is roasting, blend or food process the rosemary citrus butter ingredients together. At the 50 minute mark, carefully and evenly brush on the rosemary citrus butter. If at any point the turkey skin starts to look too brown, you can tent it with foil and then remove the foil at the end to get the crispy brown skin. When the turkey is cooked, remove from the oven and let rest for 15-20 minutes.