In a large heavy bottomed saucepan, or stockpot, warm the olive oil over medium heat. When hot, add the onions, and begin to soften, stirring frequently, this should take 15-20mins.
Once softened, turn the heat up to medium high, add the butter, and cook the onions until they are golden brown, about another 10-15mins.
Add the sugar, this will help with the browning (caramelisation), and a little salt. After another 5-10mins, the onions should be thoroughly browned.
Finally, add the garlic, stir for another 5mins.
Once the garlic is cooked in, add the vermouth or white wine. Use a flat/square stirrer to scrape away the stuck bits at the bottom of the pot.
The soup base is now ready to add the stock, and the herbs.
Begin to simmer the soup with a lid, aim for a low simmer with only the occasional bubbling. The soup should simmer for about 30mins, to fully develop the flavours.
Add any additional salt and/or pepper to taste, remove the bay leaves, and finally, add the brandy to complete the dish.
As the soup simmers, preheat your oven to 220˚C/450°F, place a rack in the upper third of the oven, and line a baking sheet with aluminium foil - do not use parchment or baking paper, as this could burn at high heat.
Carefully cut the baguette into thick slices, about 1” or 2.5cm, and brush evenly with olive oil on both sides.
Toast the baguette n the oven until a light brown colour, 5mins should do.
Remove from the oven, and turn over to toast the other side, adding the Gruyere/Emmental cheese before returning to the oven. Bake until the cheese begins to bubble and get golden-brown spots.
Once the soup has simmered, and the cheese topped baguette is ready, serve by ladling soup into serving bowls, and place one to two slices of cheesy baguette on top.
You can also finish this soup in the oven: use individual oven-proof soup bowls, or a single large pot. Pour the soup into the bowl, top with the baguette when it’s only toasted on one side, un-toasted side up, scatter the cheese over the baguette and then bake or broil/grill until the cheese is golden-brown and bubbling, about 10mins.