Preheat convection oven to 375°F.
Place turkey bones, carrots, onions, and garlic cloves in a large roasting pan, drizzle with olive oil, season with salt and pepper, and roast for 1 hour.
Flip bones over and roast for another 20 minutes.
Dump all contents from roasting pan into a large slow cooker along with a few spears of celery. Add the 1/4 cup of Apple Cider Vinegar and a pinch or two of additional salt.
Meanwhile, pour enough water to cover the bottom of the roasting pan by an inch or so. Cook on stove top on high or medium high.
Deglaze the pan by stirring and scraping until all the bits are removed from the bottom of the pan.
Pour all of this into the slow cooker along with the turkey carcass and roasted veggies along with fresh celery, cover with water up to about 1/4” from the top of the slow cooker. Set temp on high and cook for 12-24 hours.
Ater it’s cooked let it cool for a bit, but while it’s still warm, strain through a seive lined with cheesecloth.