2Trim root end off onion. Slice or quarter the onion, peel and all. Scrub carrots and chop into 1-inch chunks. In a large shallow roasting pan, place soup bones, onion, and carrots. Bake, uncovered, about 30 minutes or until the bones are well browned, turning occasionally.
3Drain off fat. Place the browned bones, onion, and carrots in a large soup pot or Dutch oven. Pour 1/2 cup water into the roasting pan and rinse. Pour this liquid into soup pot.
4Scrub the potato and chop it into chunks, peel and all. Chop the celery stalks into thirds. Add celery, tomato, parsnip, potato, peppercorns, parsley (including stems), bay leaf, salt, thyme, and garlic to the pot. Pour in the 12 cups of water.
5Bring mixture to a boil. Reduce heat. Cover and simmer for 5 hours. Strain stock. Discard meat, vegetables, and seasonings.
6To clarify stock for clear soup: In order to remove solid flecks that are too small to be strained out with cheesecloth, combine 1/4 cup cold water, 1 egg white, and 1 crushed eggshell. Add to strained stock. Bring to a boil. Remove from heat, and let stand 5 minutes. Strain again through a sieve lined with cheesecloth.