Preheat oven to 400°F.
Place chicken on rimmed baking sheets in a single layer. Roast until deeply browned and caramelized, about 45-60 minutes.
Remove baking sheets from oven and immediately pour 1/2 cup water onto each sheet. Tip water and bones into a large, tall soup pot, scraping off any bits still stuck to the sheets.
Top with remaining ingredients and cover with cold water (at least 12 cups). Bring to a boil, then reduce heat and simmer, pushing down any exposed items at least 8 hours and up to 15 hours. If using a slow cooker, heat on high until bubbling, then reduce heat to low and cook 8-15 hours.
Remove from heat and let cool 10 minutes. Strain through a fine mesh or metal strainer into a large measuring cup or lidded bowl and stir in salt to taste.
Let sit until fat rises to the top, then skim and discard fat. Or, for easier removal, chill completely and spoon off and discard solidified fat.