Roast the Bones (Optional but Recommended)

Preheat oven to 425°F. Arrange bones, onions, carrots, and celery on large baking sheets. Roast for 45 minutes, turning once halfway through, until browned and caramelized.

Transfer the roasted bones and vegetables into your 18-quart electric roaster. Add garlic halves, bay leaves, parsley, thyme, vinegar, and 2 tablespoons of the kosher salt.

Pour in enough cold water to cover the bones completely, leaving a few inches of headspace. Stir to distribute the salt and seasonings.

Cover and set your roaster to 250°F. Let it cook low and slow for 18–24 hours.

During the first few hours, skim any foam or impurities that rise to the surface. After about 8–10 hours, taste the broth and add the remaining 1 tablespoon of kosher salt, or adjust to your liking. Continue cooking until the broth is deep in color and flavor.

Using tongs, remove large bones and vegetables. Carefully pour the broth through a fine-mesh strainer or cheesecloth into large stockpots or heat-safe containers.

Let the broth cool completely, then refrigerate overnight. The next day, remove the hardened fat layer from the top (save it for cooking, if desired).