If you roast the turkey on a sheet pan, what happens to all the turkey juice? Or is there less with this method because most juice stays in the turkey..I'm pretty committed to dry brine + spatchcocked just can't figure out this part
You will still collect some juices and rendered fat collecting under the turkey. Since the bird will be roasted skin-side up, the skin will stay crisp. If the turkey has been spatchcocked and is not stuffed, it will cook evenly and in less time than a stuffed bird. You are less like to overcook it, which means that the meat, especially the breast meat, will not be dried out. So make sure that your sheet pan has a high enough rim to hold any renderings, you will still be able to collect a fair amount of liquid for gravy.
I've done it for the past couple of years, and highly recommend it. I use a 1/2-sheet pan with an inch-ish rim. I've not had any overflow problems and did a pretty big bird last year (18-19 lbs). I put the bird on a bed of onion and orange slices (see https://food52.com/recipes...) instead of a rack. You definitely need to use care when removing the pan from the oven, because it's fairly easy to tip and spill at that point. It leaves enough drippings for making gravy, but they can be pretty salty, so take that into account. On another note I recommend having your butcher do the butterflying for you (making sure they give you the backbone). I think it's hard to get through the pelvic girdle, especially on a larger bird.
To add to HLA's advice: Have the butcher remove the wishbone, too. Much easier to carve that way. (I do that, too, routinely when I butterfly chickens for roasting.) About the gravy: roast the back separately -- I do it a few days before Thanksgiving -- and make stock with the roasted back, and gravy with the drippings, but do not season it. When reheating, add small amounts of the drippings from the roasted bird, testing several times to make sure the gravy does not become too salty. Make-ahead gravy instructions here: https://food52.com/recipes... ;o)
I did this last year. It turned out well! I hope this is helpful. (just click on the "Turkey" in the contents.) http://theroamingkitchen.net/thanksgiving-series-part-1-my-menu-game-plan/
You can still roast the spatchcocked bird in a traditional roasting pan with higher sides, if you are concerned, it will brown perfectly well.
I have been spatchcocking our turkey for years. I don't think I'll ever cook a traditional turkey again. I always make a large amount of turkey stock ahead of time with turkey wings to add to the dressing, help with the gravy and to make soup with the leftovers. I use the back, neck and wing tips to fortify the premade stock for the gravy. I usually cook the turkey on a rack set over onions, carrots and celery to keep the drippings moist so they don't burn in the wider pan at the higher heat. I keep a turkey baster close by in case the half sheet becomes too full, but it never has. Great way to cook a turkey.