6 Tbsp butter, divided
1 Tbsp olive oil
3 lbs medium yellow onions, peeled and thinly sliced
1 tsp. Sugar
1 Tbsp. flour
8 cups beef stock
2 cups dry white wine
Freshly ground black pepper
8 oz. Gruyère or Emmenthal, shredded
Melt 3 Tbsp of the butter and the oil in a large heavy pot over medium-low heat. Add onions, cover and cook, stirring occasionally, until soft and translucent, about 20 minutes. Increase heat to medium-high, uncover, add the sugar, and season to taste with salt. Sauté, stirring often, until onions are very soft and a deep golden brown.
Reduce heat to medium, sprinkle in flour, and cook, stirring constantly, for 2-3 minutes. Add about 2 cups of stock and stir to blend, then add remaining 6 cups of stock and the wine. Season to taste with salt and pepper and simmer for about 30 minutes. Adjust seasonings. Preheat oven to 425F. Meanwhile, slice the read into at least 8 thick slices. Butter both sides of the bread with the remaining 3 Tbsp of butter, then toast until golden brown on both sides in the oven.
Place a slice of toast in each of 8 ovenproof bowls, then fill bowls with the onion soup. Spread a thick layer of cheese on top of soup. Set bowls in 2 baking pans. Place in the oven and bake until cheese has browned.