At the restaurant we have the luxury of being able to make our soups with wonderfully thick, rich stocks. This may be difficult to achieve at home so we suggest the addition of a little flour to help with the thickening process. If there is one item our regular guests will never let us take off the menu, this is it. The recipe can easily be halved for a smaller party.
In a large, heavy pot, heat butter and oil over medium-low heat. Add onions, cover and cook, stirring occasionally, until softened and translucent, about 20 minutes. Increase heat to medium-high and add sugar and salt; sauté, scraping up any brown bits from the bottom of the pot, until onions are softened and a deep, rich brown, about 15 minutes.
Reduce heat to medium, sprinkle with flour and cook, stirring constantly, for 2 to 3 minutes. Gradually whisk in 2 cups of the stock, then add the remaining stock and wine. Season to taste with salt and pepper. Simmer for about 30 minutes to blend the flavours. Taste and adjust seasoning with salt and pepper, if necessary.
Preheat oven to 425ºF. Divide baguette cubes among 8 individual ovenproof bowls. Fill bowls with onion soup and sprinkle each with a thick layer of cheese. Set bowls on a large rimmed baking sheet. Bake until cheese is browned, about 8 minutes. Garnish with thyme.