Place onion halves cut sides down on an ungreased griddle or over an open flame until browned, about 5 minutes.
Place the veal bones and chicken parts in a deep 7- or 8-quart kettle, stockpot, or casserole. Add 5 quarts cold water and bring to a boil. Simmer, uncovered, for 10 minutes, skimming often. When the liquid is clear, add the browned onion, leek, celery,