In a heavy stock pot, combine the water, onions, carrots, celery, leek, parsley, thyme, bay leaf and peppercorns. Bring to a boil over high heat, then reduce the heat to a very gentle simmer. Gently simmer the stock for 30 to 45 minutes.
Strain the stock through a fine sieve lined with cheesecloth, discarding the solids or saving for another use. Cool the stock, then refrigerate until needed.