Take a large enough pan and start to heat up the fat or oil
Place the bones and carcass in the pan. Optionally cut the carcass in halves
Lightly brown the bones at medium heat; now and then turn these
Add the onion in a few pieces
Add the water, the bay leaves and the pepper
Bring slowly to a simmer, keeping the lid ajar
Simmer for 1.5 - 2 hours, until the meat can be easily removed from the bones
Take the bones out and separate the meat from the bones. Discard the bones and reserve the meat
Add the meat back to the broth and add the other vegetables cut at bit size portions. If you use this broth as the base for a glace, reserve the meat for a ragout or stew and don't add vegetables
Simmer for 15 minutes more
Serve the soup. Taste will improve further overnight in the refrigerator.