Step 1: Prepare your onions by thinly slicing them. A mandoline can help achieve even slices, which is ideal for this french onion soup recipe.
Step 2: In a large, heavy-bottomed pot or Dutch oven, melt the butter over medium-low heat. Add the sliced onions and a pinch of salt. Stir to coat them thoroughly.
Step 3: Cook the onions, stirring occasionally, for 15-20 minutes until they are very soft and translucent. This initial softening is vital for the best french onion soup recipe.
Step 4: Increase the heat to medium and add the granulated sugar. Continue to cook, stirring more frequently, for 45-60 minutes. The onions should become a deep golden-brown to rich mahogany. This slow caramelization is the key to the soup's depth of flavor for any french onion soup recipe. Scrape up any browned bits from the bottom.
Step 5: Once the onions are deeply caramelized, sprinkle the flour over them. Cook, stirring constantly, for 2-3 minutes to create a light roux, which will slightly thicken your french onion soup recipe.
Step 6: Pour in the dry white wine or sherry. Deglaze the pot, scraping up all the flavorful browned bits from the bottom. Let the wine simmer for 5 minutes until mostly evaporated.
Step 7: Add the beef broth, Worcestershire sauce, bay leaf, and optional brandy. Bring the french onion soup to a gentle simmer, then reduce heat to low, cover, and cook for at least 30 minutes to allow the flavors to meld. Season with salt and pepper to taste.
Step 8: While the soup simmers, preheat your broiler. Arrange baguette slices on a baking sheet and toast them under the broiler for 1-2 minutes per side until golden. If desired, rub each toasted crouton with a cut garlic clove.
Step 9: Ladle the hot french onion soup recipe into oven-safe crocks or bowls. Float 1-2 toasted baguette slices on top of each bowl. Generously cover the bread and soup edge with shredded Gruyère cheese.
Step 10: Place the crocks on a baking sheet and broil for 2-4 minutes, watching constantly, until the cheese is melted, bubbly, and beautifully golden brown. Serve your classic french onion soup recipe immediately and enjoy!