Rinse soup bones thoroughly. Some bones may have meat attached, and you may want to make a vegetable beef soup, reserving that meat.
Roast in hot oven at 425 on flat baking sheet. Roast until all the fat is rendered, meat is cooked. Discard rendered fat.
Put roasted bones in large pot, cover with cold water, just enough to cover the bones. Add one small onion, that has been charred in frying pan. Bring to strong simmer, skim any fat that may appear. Simmer for five hours.
NOTE: If you want to make bone broth, put your bones into slow cooker and cook on low for at least 24 hours. Don't worry about using the leeks, onions and carrots, although you could.
Caramelize leeks, remaining onions, carrots by roasting at 400 for 20-30 minutes.
Add to broth, simmer for 40 minutes. Turn off heat, rest for 10 minutes.
Strain broth. Set in ice bath to cool. Refrigerate for up to 3 days. Freeze for up to two months.