Place chicken in a broiler pan or shallow roasting pan coated with cooking spray. Bake at 425° for 1 hour, turning thighs after 30 minutes.
Place chicken in a large stockpot. Carefully add 2 cups cold water to broiler pan, scraping to loosen browned bits; pour liquid into stockpot. Add onion and remaining ingredients. Add remaining 10 cups cold water; bring to a boil. Reduce heat, and simmer 2 hours; skim surface occasionally, discarding foam.
Strain stock through a fine sieve lined with cheesecloth or a thin cotton dish towel into a large bowl; discard solids. Cool stock to room temperature. Cover and chill stock for 8 hours or overnight.
Skim solidified fat from surface of stock, and discard. Place stock in a medium saucepan; bring to a boil. Reduce heat, and simmer until syrupy and reduced to 6 tablespoons (about 1 hour and 15 minutes), skimming foam as needed.