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Slow-Cooker French Onion Soup

Slow-cooked onions become soft and sweet, then get a short stovetop finish that deepens their color before beef stock, sherry, toast, and Gruyère complete the soup.

Six ceramic bowls of French onion soup are topped with toasted baguette and bubbling browned Gruyère.

French onion soup depends on onions that are sweet, soft, and deeply colored. The slow cooker handles the long collapse, while a short stint in a hot Dutch oven concentrates their flavor before the broth is added.

A little sherry lifts the beef stock, and toasted baguette with Gruyère turns each serving into its own bubbling, savory gratin.

Let the cooker collapse the onions

Four pounds of sliced onions make a generous mound, but they shrink substantially as their moisture releases. Butter, thyme, and a small amount of stock keep the strands from drying while the covered cooker gradually softens them into a sweet base.

Use the stovetop for the final color

Slow cookers soften onions well but do not create the same browned fond as a wide hot pot. Stirring the cooked onions over medium-high heat lets excess moisture escape and brings their color toward amber; keep them moving once they begin to stick so their sweetness does not cross into bitterness.

Softened onions are stirred in a Dutch oven to deepen their color before the stock is added. — Softened slow-cooker onions deepen in color in a wide pot before the stock is added.

Build a broth around the onions

Flour briefly cooked with the onions gives the broth a little body without making it thick. Sherry loosens the browned bits, and a gentle simmer with beef stock lets that concentrated onion flavor spread through the pot.

Finish each bowl under the broiler

Crisp baguette holds its shape longer than soft bread once it meets the broth. Two slices per bowl make a sturdy raft for the Gruyère, which melts into a browned cap during a short, closely watched broil.

Crisp baguette and browned Gruyère form a bubbling cap over amber onion broth. — Bubbling Gruyère and crisp toast sit above the sweet onion broth.

Slow-Cooker French Onion Soup

6 bowls

493 minutes total

Ingredients

• 4 lb yellow onions, halved and thinly sliced
• 4 tbsp unsalted butter
• 1 tsp kosher salt, divided
• 1/2 tsp freshly ground black pepper
• 1 tbsp fresh thyme leaves
• 8 cups low-sodium beef stock, divided
• 2 tbsp all-purpose flour
• 1/2 cup dry sherry
• 2 bay leaves
• 1 large baguette, cut into 12 slices 1/2 inch thick
• 2 tbsp olive oil
• 8 oz Gruyère cheese, coarsely grated

Method

Load the onions
Put the sliced onions, butter, 1/2 tsp kosher salt, thyme, and 1/2 cup of the beef stock into a 5- to 6-quart slow cooker; toss so the onion strands are lightly coated.
Look for: The butter is distributed in small pieces and the onions sit in a shallow layer of stock at the bottom.
Slow-cook until sweet
Cover and cook on low for 7 hours, stirring once around the halfway point if convenient, until the onions are very soft, reduced, and tan-gold; keep the lid closed for the rest of the cook.
Look for: The onion strands have collapsed into a soft, sweet-smelling mass with light golden edges.
Deepen the onion color
Transfer the onions and their liquid to a wide Dutch oven over medium-high heat; stir and scrape for 8 to 12 minutes as excess moisture evaporates and the onions turn a deeper amber without scorching.
Look for: The onions look glossy and deeply golden, and a spoon dragged across the pot leaves a brief track.
Thicken and deglaze
Sprinkle the flour over the onions and stir for 1 minute, then pour in the sherry and scrape up the browned fond; simmer until the sharp alcohol aroma softens, about 2 minutes.
Look for: No dry flour remains, and the sherry loosens the browned bits into a smooth, savory paste.
Finish the broth
Pour in the remaining 7 1/2 cups beef stock, add the bay leaves and black pepper, and bring the soup to a gentle simmer; lower the heat and simmer uncovered for 15 minutes so the broth takes on the onions' sweetness.
Look for: The broth gently bubbles around the edges and has a light body without a floury taste.
Toast the baguette
While the broth simmers, heat the oven to 400 F, brush both sides of the baguette slices lightly with olive oil, and arrange them in one layer on a sheet pan; bake 8 to 10 minutes, turning once, until crisp and golden.
Look for: The slices are dry and crisp through the center, with lightly browned edges.
Season and heat the broiler
Remove and discard the bay leaves, stir in the remaining 1/2 tsp kosher salt, then taste and adjust the salt if needed; heat the broiler and set six oven-safe bowls on a rimmed sheet pan.
Look for: The broth tastes savory and balanced, and the bowls are stable on the sheet pan.
Fill the bowls
Ladle the hot soup and onions evenly into the six oven-safe bowls, leaving a little headroom; float two toasted baguette slices on each bowl and cover the bread with the grated Gruyère.
Look for: Each bowl has plenty of onion, two crisp bread slices, and cheese spread to the edges.
Melt the cheese
Broil the bowls on the sheet pan, watching closely, until the Gruyère is melted, bubbling, and browned in spots, 2 to 4 minutes; use oven mitts to remove the hot bowls.
Look for: The cheese forms a bubbling cap with a few toasted brown patches and the bread remains visible at the rim.
Rest and serve
Let the bowls stand for 2 minutes so the broth settles beneath the cheese, then serve hot with a spoon and warn diners that the bowls are still very hot.
Look for: The cheese stays molten while the broth beneath it is hot enough to steam.