500 grams thinly sliced onions
2 tablespoons unsalted butter
2 tablespoons olive oil
½ teaspoon sugar
1 tablespoon all-purpose flour
1 litre strongly-flavoured vegetable stock
¼ cup dry white wine
salt and pepper
2 tablespoons cognac (optional)
1 cup (to taste) grated cave-aged gruyere cheese
toasted croutons or baguette slices
Melt the butter and oil together in a saucepan. Add the onions, toss to coat and cover the pot. Reduce the heat to low and let them cook slowly for 15 minutes. Uncover the pot, raise the heat slightly and stir in the salt and sugar. Cook onions, stirring frequently, for 30 to 40 minutes until they have turned an even, deep golden brown.
After the onions are fully caramelized, sprinkle them with flour and cook, stirring, for 3 minutes. Add the wine and then gradually stir in the stock. Season to taste with salt and pepper. Bring to a simmer and simmer partially covered for 30 to 40 more minutes. Stir in the cognac, if using.
Preheat oven to 325℉. Arrange ovenproof soup bowls on a baking sheet. Bring the soup back to a boil and divide among six bowls. Stir a tablespoon of grated cheese into each bowl, and top with croutons. Mound grated cheese on top. Bake for 20 minutes and finish for a minute or two under the broiler. Serve immediately.