Step 1

Melt the butter in a very large Dutch oven or pot over medium heat. Add the onions, bay leaves, thyme, 1 teaspoon salt, and a few grinds black pepper. Cook, stirring occasionally, until the onions are very soft and light golden-brown, about 1 hour. (The onions will begin to stick to the bottom of the pan and turn brown after about 30 minutes; just keep scraping them up and stirring them into the rest of the onions. If they are browning to quickly, lower the heat.)

Step 2

Add the wine to the pot, bring to a simmer and cook until it has almost completely evaporated, about 4 minutes. Add the beef broth and 1 cup of water, bring to a simmer and cook until the flavors come together and the soup is rich, about 45 minutes. Remove the bay leaves. Season with 1/2 teaspoon salt and a few grinds of black pepper. Keep the soup warm on a low burner or cool completely and refrigerate to reheat later. (The soup can be refrigerated for up to 2 days or frozen for up to 1 month.)

Step 3

Preheat the broiler. Place four soup crocks on a rimmed baking sheet and fill them about 3/4 of the way with hot soup. Float a slice of baguette on top of each and cover with 1/2 cup Gruyere. Broil until the Gruyere is golden brown (don't be alarmed if some soup bubbles out), 2 to 3 minutes.