Assemble and attach the KitchenAid® Rotor Slicer/Shredder attachment to the KitchenAid® Stand Mixer. Use the thick slicer cone (No. 3), turn to Speed 4 and slice onions; set aside.
Exchange thick slicer cone for fine shredder cone (No. 1). Turn to Speed 4 and shred Swiss cheese; set aside.
Melt butter in large pot over medium-high heat. Add onions and saute 15 minutes until transparent but not browned. Reduce heat and add sugar; continue cooking 20 or 25 minutes. Add broth, cover and simmer 45 minutes. Stir in sherry.
Ladle soup into individual serving bowls. Place one bread slice in each bowl and sprinkle with Swiss cheese. Place under broiler 3 minutes or until cheese melts.