Place meat and bones in a large stock pot. Add enough cold water to cover. Bring to a boil, then reduce to a very gentle simmer. Cook, skimming occasionally, until no more foam rises to the surface, about 1 hour. Add water if at any time the surface level drops below the bones. Remove any accumulated fat from the surface.
Add carrots, celery, parsnip, and bay leaf. Place onion halves directly on the trivet of a gas-stove burner over high heat or on a grill. Turn the onion with tongs as the onion becomes golden brown. If you do not have a gas stove, place the onion on a baking pan, and broil in the oven, turning as each side becomes golden brown. Add onion to stockpot. Continue to simmer until the meat is tender, about 2 hours.
Prepare an ice-water bath. Strain the stock through a fine sieve, or a cheesecloth-lined strainer, into a large bowl. Discard the solids. Transfer the bowl to the ice-water bath, and let the stock cool to room temperature.
Transfer the stock to an airtight container. Refrigerate for at least 8 hours, and up to overnight so the fat collects on the top and can be removed.
The stock may be frozen up to 4 months.