Remove outer skin from onions and slice very thinly into rings.
Melt butter in a large stock pot over low heat and add onions. Cook, stirring occasionally until onions are very soft, approximately 20 minutes.
Increase heat slightly and add sugar and salt. Continue to cook, stirring frequently until onions turn golden brown and begin to caramelize, being careful not to let them get too dark.
Add white wine and cook over medium heat for a few minutes, stirring. Add chicken broth, bay leaf, Worcestershire sauce, thyme and pepper. Simmer for 20-30 minutes on low.
Meanwhile, arrange baguette slices on a baking sheet. Toast under broiler until lightly toasted, then turn and toast other side. Remove from oven and rob garlic briskly over each piece while still warm.
Preheat oven to 400 degrees on roast setting so heat is coming from the top.
Shred cheese. Remove bay leaf from soup and ladle into 4 oven safe bowls. Top with two toasted baguette slices, pressing gently into the broth. Top with Swiss cheese.
Roast for about 10 minutes until cheese is melted and bubbling on top. Use broiler at the end if necessary to slightly brown the cheese.
Allow to cool slightly before serving.