In a soup kettle, combine the first six ingredients. Bring to a boil over medium-high heat. Reduce heat; cover and simmer for 3 hours. Strain the broth, discarding soup bones, carrot and seasoning; skim fat.
Meanwhile, in a large skillet, saute the onions in butter over medium heat for 30 minutes or until golden brown. Divide the onions among six ovenproof bowls. Ladle about 1 cup broth onto each. Top each with a slice of bread and Swiss cheese. Bake at 350° for 50-55 minutes or until golden brown. Yield: 6 servings.