Ingredients

Equipment Needed

• •Large Dutch oven or heavy-bottomed pot (6-quart minimum)
• •Wooden spoon for scraping fond
• •Oven-safe soup crocks (6 individual or 4 large)
• •Sturdy baking sheet to hold crocks
• •Box grater or food processor for cheese

Instructions

1. 1

   Slice the onions

   Halve each onion through the root, peel, and slice into thin half-moons about an eighth of an inch thick. Uniformity matters here. Uneven slices caramelize at different rates, leaving you with burnt edges and pale centers in the same pot. This is meditative work. Put on some music. Four pounds of onions will seem like an absurd mountain, but they'll cook down to a fraction of their volume.

   Yellow onions provide the best balance of sweetness and depth. Sweet onions caramelize too quickly and turn to mush. Red onions muddy the color.
2. 2

   Start the caramelization

   Melt the butter with olive oil in a large Dutch oven or heavy-bottomed pot over medium heat. The combination gives you butter's flavor with olive oil's higher smoke point. Add all the onions at once. They'll heap above the rim of your pot. Don't worry. Sprinkle with the sugar and salt, then stir to coat every strand with fat. Cover the pot and cook for fifteen minutes, stirring once halfway through. The onions will collapse, releasing their moisture and becoming translucent.

3. 3

   Build the fond

   Remove the lid and reduce heat to medium-low. Now begins the real work. Cook uncovered for 45 minutes to one hour, stirring every five minutes or so. Watch the bottom of the pot. A golden crust called the fond will develop on the surface. This is concentrated flavor. Each time you stir, scrape up this crust and incorporate it into the onions. They'll progress from pale gold to amber to deep mahogany. The kitchen will smell like heaven. The onions are ready when they're uniformly dark brown, sweet, and reduced to about two cups.

   Patience is the only secret to proper caramelization. If you rush with high heat, the sugars will burn before the onions soften. Low and slow wins this race.
4. 4

   Deglaze with wine

   Push the onions to one side of the pot, exposing the bottom. Pour in the wine and listen to it sizzle against the hot surface. Use a wooden spoon to scrape up every bit of caramelized fond clinging to the pot. This is where restaurant soups get their depth. Stir the wine into the onions and cook until the liquid evaporates completely, about two minutes. The onions will turn glossy and more intensely flavored.

5. 5

   Add stock and simmer

   Pour in the beef stock. Add the thyme sprigs and bay leaves. Bring to a gentle simmer over medium heat, then reduce to low. Let the soup bubble lazily for thirty minutes. The flavors need time to marry. Taste and adjust seasoning with salt and pepper. The broth should taste rich and deeply savory, almost meaty despite the starring role of onions. Remove and discard the thyme stems and bay leaves.

   If your stock is unsalted, you may need up to a tablespoon more salt. Season gradually and taste after each addition. You can always add more but you can't take it back.
6. 6

   Toast the croutons

   While the soup simmers, prepare your bread. Preheat the broiler. Arrange baguette slices on a baking sheet and brush both sides with softened butter. Broil until golden on both sides, watching carefully as broilers vary wildly. Rub one side of each warm toast with the cut garlic clove. The rough surface will grate the garlic, perfuming the bread without overwhelming it. Set aside.

7. 7

   Assemble the gratinée

   Position a rack six inches from the broiler element. Ladle hot soup into oven-safe crocks, filling them three-quarters full. Float one or two croutons on the surface of each, garlic side down. Pile the grated Gruyère generously over the bread, letting it cascade down the sides to touch the rim of the crock. Sprinkle Parmesan over the top. Place crocks on a sturdy baking sheet.

   The cheese touching the crock's edge is intentional. It will crisp against the hot ceramic, creating those coveted crunchy bits.
8. 8

   Broil until bubbling

   Slide the baking sheet under the broiler. Watch with the intensity of a hawk. The cheese will begin to bubble within two minutes, then develop golden and brown spots. Pull the crocks when the cheese is deeply bronzed in places and still stretching, about three to four minutes total. The soup beneath will be furiously bubbling around the edges. Use thick oven mitts. Warn your guests. Those crocks retain heat like small furnaces. Rest for two minutes before serving. This soup waits for no one.

Chef Tips

• •The quality of your beef stock determines the quality of your soup. If using store-bought, seek out products sold in glass jars at specialty grocers, or the concentrated pastes that approximate real gelatin. Boxed broths labeled 'stock' often contain more salt than substance.
• •Making stock from scratch transforms this soup entirely. Roast five pounds of beef bones at 450°F until mahogany brown, then simmer with aromatic vegetables for eight hours. The collagen converts to gelatin, giving body that no commercial product can match. Freeze in quart containers for future batches.
• •Dry sherry adds complexity that white wine cannot. Look for Fino or Manzanilla, both bone-dry styles that contribute nuttiness without sweetness. Avoid 'cooking sherry' sold in grocery stores, which contains salt and tastes of nothing worth eating.
• •The crocks matter. Thin ceramic or glass bowls crack under a broiler and don't retain heat properly. Invest in proper oven-safe soup crocks with small handles. They'll last decades and make this soup the theatrical event it deserves to be.

Advance Preparation

• •The soup base, through step 5, can be made up to four days ahead and refrigerated. The flavor deepens considerably overnight as the onions continue to release their essence into the broth.
• •Freeze the soup base without bread or cheese for up to three months. Thaw overnight in the refrigerator and reheat gently before proceeding with gratinée.
• •Croutons can be toasted and garlic-rubbed up to one day ahead. Store in an airtight container at room temperature. They'll soften slightly but crisp again under the broiler.
• •Grate the cheese up to two days ahead and refrigerate in a sealed container. Bring to room temperature before using for better melting.

Frequently Asked Questions

Nutrition Information

1 serving (about 320g)