Adapted from epicurious.com
4 2-inch slices of beef bones
4 sprigs of flat-leaf parsley
1 sprig of thyme
1 bay leaf
2 onions, unpeeled, quartered
1 carrot, peeled, quartered
2 celery ribs, quartered
1. Preheat oven to 450º. While oven heats, tie parsley, thyme and bay leaf in a bundle with kitchen string to make a bouquet garni. Spread beef bones, onions and carrot in roasting pan, then brown well in oven, turning occasionally, about 1 hour.
2. Transfer meat and vegetables to a large stockpot. Add 2 cups water to roasting pan, then deglaze by boiling over high heat, stirring and scraping up brown bits, 2 minutes.
3. Add deglazing liquid to stockpot along with 18 cups of water, celery, salt and the bouquet garni. Bring to a boil and skim froth. Add remaining 1/2 cup water, then bring mixture to a simmer and skim any froth. Simmer gently, uncovered, skimming froth occasionally, until liquid is reduced to about 8 cups, about 4 hours.
4. Pour stock through a fine-mesh sieve into a bowl, pressing hard on and then discarding solids. If using stock right away, skim off and discard fat. If not, cool stock completely, uncovered, then chill, covered (it will be easier to remove fat when chilled).