French Onion Soup Recipe

Prepare onions

Peel and thinly slice 6 medium yellow onions into half-moons so you have about 6 cups loosely packed slices.

Caramelize onions

In a large, heavy-bottomed pot or Dutch oven, heat 3 tbs unsalted butter and 1 tbs olive oil over medium heat until butter foams. Add the sliced onions, 1 tsp salt, 1/2 tsp black pepper, and 1/2 tsp granulated sugar. Toss to coat. Cook, stirring every 3–5 minutes, adjusting heat as needed to maintain a gentle but steady sizzle. Continue for 35–45 minutes until the onions are a deep golden-brown and richly caramelized. If the onions begin to stick or burn, lower the heat and scrape the fond from the bottom of the pot with a wooden spoon.

Deglaze

Raise the heat to medium-high. Pour in 3 tbs dry white wine or sherry to deglaze the pan, scraping up the browned bits from the bottom. Let the alcohol cook off for 1–2 minutes.

Add liquids and aromatics

Stir in 1 tbs tomato paste and cook 1 minute to deepen flavor. Add 6 cups beef stock and 1 cup chicken stock (if using) along with 3 sprigs thyme and 1 bay leaf. Bring to a gentle simmer, then reduce heat to low and simmer, uncovered, for 20–30 minutes to meld flavors. Taste and adjust seasoning with salt and pepper. If using, stir in 1 tbs dry sherry or cognac at the end for brightness.

Make baguette toasts

While the broth simmers, preheat the oven to 400°F (200°C). Slice the baguette into 1/2-inch thick slices. Brush both sides lightly with 1 tbs extra-virgin olive oil and arrange on a baking sheet. Toast for 6–8 minutes, flipping once, until golden and crisp. Rub the hot toasts with the cut side of 1 garlic clove for a subtle garlic scent.

Prepare cheese

Grate 2 cups Gruyere and, if using, 1/4 cup Parmesan. Mix together and set aside.

Assemble and melt

Ladle the hot soup into ovenproof bowls. Place 1–2 crostini on top of each bowl (or set the crostini on top after a short broil if you prefer them fully covered). Sprinkle a generous handful of the Gruyere mixture over each crostini, covering the toast and a little of the soup surface. Place bowls on a baking sheet and broil on high 3–5 inches from the element for 2–4 minutes, watching closely, until the cheese is bubbling, golden and slightly browned. Alternatively, bake at 425°F (220°C) for 6–8 minutes until melted and browned.

Remove bowls from oven carefully using oven mitts. Let cool 1–2 minutes—cheese will be very hot. Sprinkle with chopped fresh parsley if desired and serve immediately. Discard thyme sprigs and bay leaf from the soup before serving.