In a large stock pot or Dutch Oven, melt the butter. Add the onions and sugar. Cook over medium heat for 20 minutes until the onions are translucent.
Add in garlic, salt, and pepper and continue cooking on low heat for 1 1/2 hours, stirring occasionally to prevent the onions from sticking and burning. Cook on very, very low heat.
Use a flat-ended spoon to scrape up the brown bits. The onions are finished when they are a deep golden amber-like caramel.
Stir in the flour and cook for 1 minute. Add the wine and scrape up the brown bits. Stir in the sherry, Worcestershire, and chicken stock. (You can use vegetable stock for a vegetarian option).
Bring to a boil, then reduce heat to a simmer. Cook for another 45 minutes. Add more water (1/2 cup at a time) if the broth cooks down too quickly. Stir in balsamic vinegar.
For the Toast
Preheat oven to 400 degrees. Rub the sliced garlic halves on the bread. Toast the bread in the oven until the edges are browned- about 2-3 minutes. Rub with garlic again while the bread is still warm. Set aside.
Line a rimmed baking sheet with parchment paper and place 4 oven-proof bowls on the sheet pan.
Ladle the soup into the bowls, and top with toast and a large serving of grated Gruyére cheese on top of the toast.
Bake at 400 degrees until the cheese is bubbling and starting to brown. About 5-7 minutes.
Serve immediately.
