Restorative Beef Broth

Combine short ribs, onion, prunes, and garlic in a very large heavy pot; add 6 quarts water (preferably filtered or spring water). If unable to add all water at once, pour in remaining liquid later as broth reduces. Bring to a boil over medium-high heat, skimming any foam from the surface. Cover with lid slightly ajar. Reduce heat to low and simmer very gently, occasionally skimming the surface, until meat is tender, about 2 1/2 hours. Uncover; simmer until broth is intensely flavored, about 3 hours longer. Place a fine-mesh sieve over another large pot; line with a layer of cheesecloth and strain broth (you should have about 8 cups). Remove beef from bones in strainer and reserve for another use; discard remaining solids in strainer. Spoon fat from surface of broth. Season to taste with salt. DO AHEAD: Can be made 3 days ahead. Cover and chill. Rewarm before serving.