Originally from Lyons, onion soup was made popular during the 19th Century by Parisian market workers in Les Halles. Traditionally served after a nuit blanche (sleepless night) because of its supposed ability to quickly dissipate the effects of alcohol, it is an invigorating and revitalising soup.
Heat the butter in a large, deep, heavy-based saucepan. Gently fry the onion and garlic for about 15 minutes. They must be soft, brown and lightly caramelised. Be careful not to let them fry crisp.
Add the wine, stock, thyme and bay leaves. Bring to the boil, then reduce the heat and simmer very gently for around 30 minutes, stirring from time to time.
Preheat the grill. Cut the baguette into 2cm slices and toast it on both sides. Pour the soup into ovenproof bowls, place 1 or 2 pieces of baguette on top and sprinkle generously with the grated Gruyère.
Put the bowls under the hot grill. When the cheese is golden and bubbling, remove from the oven and serve immediately.