Quick Recipe Version (TL;DR)
Quick Ingredients
- 3 lb yellow onions, thinly sliced (about 5–6 medium)
- 4 tbsp unsalted butter
- 2 tbsp olive oil, divided
- 1 1/2 tsp kosher salt, divided, plus more to taste
- 1/2 tsp freshly ground black pepper
- 1/2 cup dry sherry
- 6 cups low-sodium beef broth
- 6 fresh thyme sprigs (or 1 tsp dried)
- 1 bay leaf
- 1 tsp Worcestershire sauce (optional)
- 12 baguette slices, 1/2-inch thick
- 8 oz Gruyère, grated (about 2 1/2 cups lightly packed)
- 2 tbsp finely grated Parmesan (optional)
Do This
- 1) Caramelize onions: Melt butter with 1 tbsp oil in a heavy pot over medium heat. Add onions, 1 tsp salt. Cook, stirring, until deep mahogany, 45–60 min.
- 2) Deglaze: Stir in sherry; scrape brown bits; reduce by half, 2–3 min.
- 3) Simmer: Add broth, thyme, bay, pepper, and Worcestershire. Simmer 30 min; season to taste.
- 4) Toast bread: Brush baguette with 1 tbsp oil; broil on a sheet until crisp, 1–2 min per side.
- 5) Preheat broiler to High (500°F/260°C). Set rack ~6 inches from element.
- 6) Assemble: Ladle soup into broiler-safe bowls, float toasts, mound with Gruyère (and Parmesan).
- 7) Broil 3–5 min until bubbling and browned. Rest 5 min; serve hot.
Why You’ll Love This Recipe
- Deep, restaurant-level flavor from slowly caramelized onions and a gentle sherry deglaze.
- Herbaceous backbone from fresh thyme and bay, balanced by rich beef broth.
- A dramatic, crackly Gruyère cap that melts, bubbles, and browns beautifully.
- Clear, make-ahead friendly steps so you can finish under the broiler right before serving.
Grocery List
- Produce: Yellow onions (3 lb), fresh thyme, bay leaf
- Dairy: Unsalted butter, Gruyère, Parmesan (optional)
- Pantry: Olive oil, low-sodium beef broth, dry sherry, Worcestershire sauce (optional), kosher salt, black pepper, baguette
Full Ingredients
Onion Base
- 3 lb yellow onions, halved root-to-stem and thinly sliced
- 4 tbsp unsalted butter
- 1 tbsp olive oil
- 1 tsp kosher salt (to start)
Deglaze & Broth
- 1/2 cup dry sherry
- 6 cups low-sodium beef broth
- 6 fresh thyme sprigs (or 1 tsp dried)
- 1 bay leaf
- 1 tsp Worcestershire sauce (optional, for depth)
- 1/2 tsp freshly ground black pepper
- Kosher salt, to taste (broth saltiness varies)
Gruyère Toast Cap
- 12 baguette slices, 1/2-inch thick
- 1 tbsp olive oil (for brushing bread)
- 8 oz Gruyère, coarsely grated (about 2 1/2 cups lightly packed)
- 2 tbsp finely grated Parmesan (optional, for extra browning)
Step-by-Step Instructions
Step 1: Prep the onions
Halve onions from root to stem, peel, then slice thinly (about 1/4-inch). This shape helps onions hold together during the long cook. Gather your thyme sprigs and bay leaf so they are ready when you add the broth.
Step 2: Slowly caramelize
In a heavy 5–6 quart Dutch oven, melt the butter with 1 tablespoon olive oil over medium heat. Add onions and 1 teaspoon kosher salt. Cook, covered, for 10 minutes to jump-start softening. Uncover, reduce heat to medium-low, and continue cooking 35–50 minutes, stirring every few minutes. Regulate heat so the onions deepen to a uniform mahogany without scorching. If browned bits build on the bottom, splash in 1–2 tablespoons water and scrape with a wooden spoon to dissolve them back in.
Step 3: Deglaze with sherry
Increase heat to medium. Add the dry sherry and stir, scraping up any fond. Let it bubble and reduce by about half, 2–3 minutes. This concentrates the wine’s nutty, slightly sweet flavor and lifts every last bit of caramelization from the pot.
Step 4: Build and simmer the soup
Add beef broth, thyme sprigs, bay leaf, Worcestershire (if using), and black pepper. Bring to a gentle boil, then reduce to a steady simmer and cook 30 minutes. Fish out the thyme and bay. Taste and adjust seasoning with additional salt and pepper as needed. If the broth tastes too salty, balance with a splash of water.
Step 5: Toast the baguette
Position an oven rack about 6 inches below the broiler element. Arrange baguette slices on a baking sheet and brush lightly with 1 tablespoon olive oil. Broil on High until golden and crisp, 1–2 minutes per side. Set aside. While the bread toasts, coarsely grate the Gruyère and finely grate the Parmesan (if using).
Step 6: Assemble and broil the Gruyère cap
Preheat the broiler to High (about 500°F/260°C). Ladle hot soup into broiler-safe bowls set on a rimmed baking sheet. Float 1–2 toasts on each. Mound generously with Gruyère, letting some cheese overlap the edges so it melts into a sealed “cap.” Sprinkle Parmesan over the top if you like extra browning.
Broil until the cheese is fully melted, deeply browned in spots, and bubbling, 3–5 minutes. Rotate the pan once for even color if needed.
Step 7: Rest and serve
Let the bowls stand 5 minutes so the molten cheese sets slightly. Serve hot, optionally garnished with a few thyme leaves and a twist of black pepper.
Pro Tips
- Use a wide, heavy pot: More surface area means faster, more even caramelization.
- Manage moisture: If onions threaten to scorch, lower the heat and add a tablespoon of water to dissolve fond.
- Sherry matters: Dry sherry adds nutty depth; avoid sweet styles. Reduce briefly to cook off harsher alcohol notes.
- Pre-toast for structure: Crispy toasts won’t collapse under the weight of cheese and stay pleasantly chewy after broiling.
- Broiler safety: Only use broiler-safe bowls or crocks. Set them on a sheet pan for easier handling.
Variations
- Comté or Emmental Cap: Swap part or all of the Gruyère for Comté or Emmental for a slightly fruitier, nutty finish.
- Madeira or Cognac Twist: Replace half the sherry with Madeira or add 1 tablespoon cognac off heat just before ladling for a luxurious aroma.
- Vegetarian Umami: Use rich mushroom stock instead of beef broth and add 1 teaspoon white miso to the simmer for depth.
Storage & Make-Ahead
The soup base (without bread and cheese) keeps 4 days in the refrigerator or up to 3 months in the freezer. Cool completely, store airtight, and reheat gently until simmering. Toast the bread and broil with cheese just before serving for best texture. Leftover assembled bowls do not store well—cheese and bread become soggy upon reheating—so build only what you plan to serve.
Nutrition (per serving)
Approximate: 480 calories; 24 g fat; 45 g carbohydrates; 3 g fiber; 18 g protein; 980 mg sodium. Values will vary based on broth and cheese brands and the number of toasts per bowl.
