Preheat oven to 200 degrees C. Roast bones on an oven tray, uncovered, about 1 hour or until browned.
Combine bones, onion, carrot, celery, half the water, bay leaves and peppercorns in large saucepan or boiler; bring to a boil. Reduce heat; simmer, uncovered, 2 hours, skimming surface occasionally.
Add remaining water; simmer, uncovered, 1 hour. Strain stock through muslin-lined sieve or colander into a large heatproof bowl; discard solids. Allow stock to cool, cover; refrigerate until cold. Skim and discard surface fat before using.