Place a large heavy-bottom pot over medium heat. Add the butter and let it melt. Once the butter has melted, add your onions, salt, thyme, and bay leaves. Reduce the heat to low and cover.
¼ cup unsalted butter, 7 yellow onions, 1 tsp kosher salt, 3-4 sprigs fresh thyme, 2 bay leaves
Check on your onions and give them a stir every 15 minutes or so. We want to cook these onions until they turn amber and are practically mush. This can take up to 2 hours if you really want to develop that rich onion flavor. But you can pull the onions earlier if you prefer a lighter tasting soup.
While your onions are caramelizing preheat the oven to 300°F. Slice a baguette and arrange the slices on a baking sheet lined with parchment paper. ***** Bake for 30 minutes.
1 small baguette
When the onions look like they have about 15 minutes left in their cook time, slice 4 cloves of the garlic thinly and add them to the pot.
6 cloves garlic
Once the onions are where you like them and they are starting to catch on the bottom of the pot, deglaze the pot with the vermouth. Fish out and discard the thyme and bay leaves. Stir in the mustard followed by the stock. Bring the soup up to a boil then reduce to a simmer and cook for 15 minutes.
¼ cup dry vermouth **, 2 tbsp Dijon mustard ***, 6 cups good quality stock ****
While the soup is on its final simmer, place 4 onion soup bowls on a baking sheet and pop them in the oven. Turn the broiler on. While the broiler is heating up, rub the remaining garlic on each crouton.
6 cloves garlic
Once the soup is done, taste and season with additional salt if necessary. Take the soup bowls out of the oven and place 2-3 croutons into the bottom of the bowl. Ladle the soup on top and the croutons should rise to the surface. Cover the bowls with the shredded gruyere and return them to the oven. Broil the soup until the cheese melts and browns slightly. Simply garnish the soup with a sprig of fresh thyme and serve.
185g (6.5oz) gruyere