Step 1Place the bones in a large, heavy stock pot. Add the onion, carrot and celery, along with the leek, thyme, parsley, peppercorns and garlic, if using. Pour over the water.
Step 2Bring the mixture to a boil over high heat, then reduce the heat to a very gentle simmer. Skim the surface of the stock as needed, discarding the foam, impurities and fat that rise to the surface. Simmer the stock for 3 to 5 hours, adding water if needed to keep the bones submerged.
Step 3Strain the stock through a fine sieve lined with cheesecloth, discarding the solids. Cool the stock, then refrigerate until needed (be sure to remove and discard any fat that has solidified at the top of the stock before using).