Step 1Place the bones in a large (10-quart) stockpot. Make a bouquet garni by placing the parsley, thyme and bay leaf inside the leek halves and tying them with kitchen twine, leaving one end of the string about 24 inches long. Add the bouquet garni to the pot, tying the long end of the string to the pot. Add the onions, celery, carrots and peppercorns.
Step 2Add 5 quarts cold water and cook over high heat until the water comes to a simmer, about 30 minutes. Immediately reduce the heat to low so that the stock is barely simmering.
Step 3Simmer uncovered over low heat for about 4 hours, skimming the top to take off any scum and being careful not to let the stock boil. If it does, move the pan slightly to the side of the burner.
Step 4After 4 hours, strain the stock into a large pan through a large fine-mesh strainer. Put this pan in the sink, and fill the rest of the sink with ice cubes and cold water so the stock cools quickly. Once cool, cover and refrigerate.
Step 5When the stock is cold, remove and discard the layer of fat from the surface. Refrigerate or freeze the stock until needed.