French Onion Soup Au Boeuf Recipe
Coal-roasted caramelized onions, beef tenderloin, veal cognac broth, rosemary croutons, gruyere
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Ingredients
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Onions & Aromatics
Coal-Roast Step (optional but recommended)
Broth & Deglaze
Beef Tenderloin
Croutons & Cheese
Finishing & Garnish
Method
Onions & Aromatics
- 1
Prepare onions
Peel and thinly slice the yellow onions into half-moons about 1/8-inch thick. Measure approximately 6 cups sliced.
- 2
Coal-roast the onions (optional but adds charred flavor)
If using a grill or charcoal, preheat until white-hot coals form. Lightly brush onion slices with 1 tsp vegetable oil. Place onions on a perforated grilling basket or on a sheet of aluminum with holes and briefly hold over coals until edges develop deep char and some pieces are blackened, about 1–2 minutes per side. The goal is spots of coal-roast color, not fully cooking them. Transfer to a bowl.
- 3
Caramelize onions
In a large heavy-bottomed pot or Dutch oven, heat 2 tbs olive oil and 3 tbs unsalted butter over medium heat until butter foams. Add the prepared onions, 1.5 tsp fine sea salt, 1/2 tsp black pepper and 1/2 tsp granulated sugar. Stir to coat. Cook gently, stirring every 5–7 minutes, lowering heat if onions start to burn. Continue until deep golden-brown and richly caramelized, about 45–60 minutes. If you used the coal-roast step, start with medium heat and shorten total caramelization time (about 30–45 minutes) because of initial char and softened texture.
Deglaze & Make Broth
- 4
Deglaze with wine
Increase heat to medium-high. Add 3/4 cup dry white wine or sherry to the caramelized onions to deglaze the pot, scraping up any browned bits on the bottom with a wooden spoon. Simmer until the wine reduces by about half, 3–4 minutes.
- 5
Add stocks and aromatics
Add 6 cups veal stock, 1/2 cup beef demi-glace, 2 tbs cognac, 3 thyme sprigs, 1 rosemary sprig and 1 bay leaf. Bring to a gentle simmer, reduce heat to low, and simmer uncovered for 20–30 minutes to meld flavors. Taste and adjust seasoning; add 1 tsp sherry vinegar or a splash of sherry if you want brightness.
- 6
Strain (optional for a clearer broth)
If you prefer a smoother broth, remove thyme sprigs, rosemary and bay leaf, then strain the broth through a fine-mesh sieve into a clean pot, returning the onions to the broth or discarding them per preference. For classic French onion soup, leave the onions in the pot.
Beef Tenderloin
- 7
Season and sear beef
Pat 10 oz beef tenderloin pieces dry and season with 1/2 tsp salt and 1/4 tsp freshly ground black pepper. Heat 1 tsp neutral oil in a skillet over high heat until shimmering. Sear the beef briefly, 30–60 seconds per side for rare to medium-rare depending on slice thickness — you want browned exterior but not overcooked interior. Remove and set aside on a plate; the beef will finish warm in the soup bowls.
- 8
Add beef to bowls
When assembling, divide the seared beef evenly among the serving bowls so each bowl receives tenderloin pieces under the cheese-topped crouton.
Croutons & Cheese
- 9
Prepare rosemary croutons
Preheat oven to 425°F (220°C). Arrange 8 baguette slices on a baking sheet. Brush each slice lightly with 2 tsp olive oil (divide across slices) and sprinkle with 1 tsp finely chopped fresh rosemary. Bake until golden and crisp, 6–8 minutes, flipping once. Rub each warm slice with the cut side of 1 halved garlic clove for subtle garlic flavor.
- 10
Assemble and melt cheese
Ladle hot soup into ovenproof bowls (leave room for crouton and cheese). Place seared beef pieces into each bowl. Float one rosemary crouton on top of each bowl, then pile 1/4 cup grated Gruyère (about 1/2 cup per two bowls; total 2 cups for recipe) generously over the crouton so cheese also touches the bowl rim. Sprinkle a little Parmesan if using.
- 11
Broil until bubbly and browned
Place bowls on a baking sheet and broil in the oven 4–6 inches from the heat element on high until the cheese is bubbly and golden brown, 2–4 minutes. Watch closely to avoid burning. Alternatively, use a kitchen torch to brown the cheese evenly.
Finish & Serve
- 12
Final seasoning and garnish
Remove bowls from oven carefully (they will be hot). Sprinkle a tiny pinch of coarse sea salt and freshly ground black pepper if needed. Garnish with a small rosemary tip or thyme leaf if desired.
- 13
Serve
Serve immediately with extra crusty bread. Caution diners about hot bowls and molten cheese.