Cut onions into wedges, starting at top and cutting to within 1/2 inch of root end. Separate wedges slightly, and place in a lightly greased, large roasting pan. Add turkey parts.
Bake at 500° for 45 minutes or until browned and turkey is done.
Bring turkey, onions, 5 cans broth, carrot, and next 4 ingredients (celery through fennel seeds) to a boil in a large Dutch oven; cover, reduce heat, and simmer 1 hour. Remove from heat; cool. Remove and discard bay leaf.
Drain vegetables and turkey, reserving broth in Dutch oven. Process vegetables in a food processor until smooth. Skin, bone, and finely chop turkey.
Skim foam from broth. Return turkey and vegetable mixture to broth. Add bread and remaining can chicken broth, if needed. Cook over medium-high heat, stirring occasionally, 30 minutes. Mixture will be slightly lumpy.