Soup:

1. In a large pot, approximately 8 qt, set over medium heat, add the butter. Once melted and bubbling, add the onions and immediately season with salt. Stir to evenly coat. Cover with a lid and let the onions sweat for 10–15 minutes, stirring occasionally. Remove the lid and continue cooking, stirring frequently, until deeply caramelized, about 45–60 minutes. If the onions start to get too dark before they are fully caramelized, deglaze with a splash of water and continue cooking until evenly caramelized. Adjust the heat as necessary to prevent burning.
2. To make the sachet, place the coriander seeds, black peppercorns, and thyme sprigs in a piece of cheesecloth. Gather the edges, form a bundle, and tie securely with kitchen twine.
3. Stir in the flour and cook for 1 minute. Increase the heat to medium-high, then deglaze with the bourbon and white wine. Bring to a boil and reduce the liquid to approximately 70% of its original volume. Add the beef stock and bring to a boil, then reduce the heat to maintain a simmer. Add the sachet and simmer for 15 minutes. Remove the sachet.
4. Finish with the shirodashi and sherry, then season to taste with additional salt, if needed. Whisk in the cold, cubed butter until fully incorporated.

Assembly:

1. In a large bowl, toss the baguette slices with the melted butter, flaky salt, and finely chopped thyme. Transfer to a foil-lined baking sheet and bake in a preheated 425°F (218°C) oven until crisp but not browned, about 5–7 minutes.
2. Combine the Gruyère and Gouda in a medium bowl, mixing until evenly distributed.
3. In an oven-safe bowl, ladle in the soup until it reaches ¼ inch below the rim. Top with 2–3 slices of toasted bread and a generous amount of the cheese mixture. Place under a broiler set to high and broil until the cheese is browned and bubbling, about 2–4 minutes. Garnish with chives. Serve and enjoy!