Caramelize the Onions
In a large Dutch oven, melt the butter with the oil over medium heat. Add the onions, salt, pepper, and sugar. Cook uncovered, stirring occasionally at first, then more frequently as the onions brown. Continue for 45–55 minutes, until deeply golden and caramelized. If they start browning too fast, lower the heat or add a splash of water to deglaze.

Deglaze with Wine
Pour in the white wine, scraping up the flavorful brown bits from the pot. Let the wine cook down until it’s almost evaporated and the onions are glossy and jammy, about 8–10 minutes.

Add Flour
Sprinkle in the flour and stir constantly for one minute. This gives the soup body and richness.

Simmer the Broth
Stir in the beef broth, Worcestershire sauce, thyme, and bay leaves. Bring to a boil, then reduce heat and simmer gently for 30 minutes, allowing the flavors to blend. Discard the bay leaves and stir in the dry sherry.

Toast the Bread
While the soup simmers, preheat your oven to 400°F (200°C). Arrange baguette slices on a baking sheet and bake until golden and crisp, about 10 minutes. Set aside.

Assemble the Crocks
Set the oven rack 6 inches from the broiler. Place oven-safe soup crocks on a baking sheet. Ladle hot soup into each, top with one or two baguette slices, and sprinkle generously with the Gruyère and Parmigiano Reggiano.

Broil and Serve
Broil for 3–5 minutes, until the cheese is melted, bubbly, and lightly golden. Let cool briefly before serving—your French Onion Soup will be hot, creamy, and perfectly cheesy.