Onion Soup Au Gratin Recipe

Prepare onions

Peel and cut the onions in half from root to tip, then thinly slice each half into half‑moon slices so you have about 1.2 kg total.

In a large heavy-bottomed pot or Dutch oven, melt the butter with the olive oil over medium heat until foaming. Add the sliced onions and toss to coat.

Cook the onions over medium to medium-low heat, stirring every few minutes. After about 10 minutes, sprinkle the sugar over the onions to help with caramelization. Continue to cook for 30–40 minutes until the onions are deep golden brown and sweet, reducing heat if they start to stick or burn. Patience is key: evenly browned onions give the best flavor.

Add the minced garlic and cook 1 minute more until fragrant, stirring so it doesn't brown.

Sprinkle the flour over the onions and stir to incorporate. Cook 2–3 minutes to remove the raw flour taste; this will help slightly thicken the soup.

Deglaze the pot with the white wine, scraping any browned bits from the bottom with a wooden spoon. Bring the wine to a simmer and reduce by half, about 2–4 minutes.

Add the beef broth and chicken broth, bay leaves, thyme sprigs, Worcestershire sauce, salt, and pepper. Stir to combine and bring to a gentle simmer. Reduce heat to low and simmer, uncovered, for 20–30 minutes to let flavors meld.

Taste and adjust seasoning with additional salt and pepper if needed. Remove and discard bay leaves and thyme stems before serving.

While the soup simmers, preheat the oven to 220°C (425°F) with the rack in the upper third so the cheese browns nicely.

Arrange the baguette slices on a baking sheet. Brush each slice lightly with the extra virgin olive oil and toast in the oven until crisp and just golden, about 5–7 minutes. Flip halfway for even toasting. Alternatively, use a broiler for 1–2 minutes per side—watch carefully.

Ladle the hot soup into ovenproof bowls (ramekins or crocks), filling each about three-quarters full to allow room for the bread and cheese. Place one or two toasted baguette slices on top of each bowl of soup.

Top each bread slice generously with grated Gruyère (about 60–80 g per serving) and a light sprinkle of grated Parmesan if using.

Place the filled bowls on a baking sheet and transfer to the oven. Broil or bake until the cheese is bubbling and golden brown, 3–6 minutes depending on your oven and distance to heat source. Rotate the sheet if needed for even browning. If using a 220°C oven instead of broil, bake 8–10 minutes.

Carefully remove the baking sheet and let the bowls sit 1–2 minutes. Garnish with fresh thyme leaves and a grind of black pepper. Serve immediately with additional toasted bread on the side.