Melt the butter and olive oil in a large heavy-bottomed pot or Dutch oven over medium heat. Add the sliced onions, salt, and sugar. Toss to coat the onions in the fat.

Cook the onions, stirring occasionally, until they reduce in volume, soften, and develop a deep mahogany color. If the bottom of the pot gets too dark, add a splash of water and scrape up the browned bits.

Once the onions are deeply caramelized, add the minced garlic and cook for 1 minute until fragrant. Sprinkle the flour over the onions and cook, stirring constantly, for 2 minutes to cook out the raw flour taste.

Pour in the dry sherry to deglaze the pot, scraping up any fond (browned bits) stuck to the bottom. Cook until the sherry has mostly evaporated.

Stir in the beef broth, thyme sprigs, bay leaf, and black pepper. Bring the soup to a boil, then reduce the heat to low, cover partially, and let simmer to meld the flavors.

While the soup simmers, preheat the oven to 400°F (200°C). Arrange the baguette slices on a baking sheet and toast for 5-7 minutes until dry and lightly golden. Remove from the oven and switch the oven setting to Broil.

Place 4 oven-safe soup crocks on a sturdy baking sheet. Ladle the hot soup into the bowls, leaving about an inch of space at the top. Top each bowl with 1-2 slices of toasted baguette, then generously pile the grated Gruyère cheese over the bread and up to the edges of the bowls.

Carefully transfer the baking sheet with the bowls to the oven. Broil for 3-5 minutes, watching closely, until the cheese is bubbling, melted, and golden brown in spots. Let sit for 5 minutes before serving (it will be extremely hot).