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Crock of French onion soup topped with bubbly browned Gruyère and toasted baguette, served hot

French Onion Soup with Deeply Caramelized Onions

French onion soup built on slowly caramelized onions, a deep beef broth, and a bubbly Gruyère topping, made without wine or sherry. White grape juice and cider vinegar supply the sweetness and tang.
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Prep Time 20 minutes
Cook Time 1 hour 40 minutes
Total Time 2 hours
Course: Soup
Cuisine: French
Calories: 450
Ingredients Equipment Method Notes

Ingredients
  

For the soup
  • 4 tablespoons unsalted butter
  • 1 tablespoon olive oil
  • 3 pounds yellow onions peeled and sliced 1/4 inch thick (about 6 large)
  • 1 teaspoon Diamond Crystal kosher salt
  • 3 garlic cloves minced
  • 2 tablespoons all-purpose flour
  • 1/2 cup unsweetened white grape juice
  • 1 tablespoon apple cider vinegar
  • 6 cups low sodium beef broth
  • 4 fresh thyme sprigs
  • 2 bay leaves
  • Freshly ground black pepper to taste
For the topping
  • 6 baguette slices 1/2 inch thick (about 1 ounce each)
  • 8 ounces Gruyère cheese shredded (about 2 cups)

Equipment

  • 6 to 8 quart Dutch oven
  • Flat edged wooden spoon
  • Six oven safe soup crocks (10 to 12 ounces each)
  • Rimmed baking sheet
  • Instant-read thermometer

Method
 

  1. Melt the butter with the olive oil in the Dutch oven over medium heat for 2 minutes, until the foam settles. Add the onions and salt and stir until every slice looks glossy.
  2. Cook over medium heat for 20 minutes, stirring every 5 minutes, lid off, until the onions shrink by half and turn soft and see through.
  3. Lower the heat to medium low and cook 25 to 35 minutes more, stirring every 3 to 4 minutes, until the onions turn deep amber and a brown film coats the pot bottom. When the film forms, add 2 tablespoons water and scrape it loose.
  4. Stir in the garlic and flour and cook for 2 minutes, until no dry white streaks remain.
  5. Pour in the grape juice and vinegar and scrape the pot for 2 to 3 minutes, until the liquid cooks down to a sticky glaze.
  6. Add the broth, thyme, bay leaves, and pepper. Bring to a boil over high heat, then simmer uncovered for 30 minutes, until the broth turns the color of strong tea. Taste at minute 25 and add 1/4 teaspoon salt only if it tastes flat.
  7. While the soup simmers, heat the oven to 400°F. Toast the baguette slices on the baking sheet for 8 to 10 minutes, flipping once, until they feel dry like croutons and look light gold.
  8. Remove the thyme and bay leaves. Set the crocks on the baking sheet and ladle in the soup, leaving 1/2 inch of space. Top each with a toast slice and a heaping 1/3 cup Gruyère, letting the cheese touch the rim, until it forms a thick blanket.
  9. Switch the oven to broil on high with a rack 6 inches below the element. Broil for 3 to 5 minutes, watching closely, until the cheese bubbles and shows dark brown spots.
  10. Use oven mitts to move the crocks to a rack and let them stand for 5 minutes, until the cheese stops bubbling and sets into a stretchy, golden lid.

Notes

Diamond Crystal is lighter by the teaspoon than Morton. With Morton kosher salt, use 1/2 teaspoon.
No grape juice? Add 1/2 cup more broth and 1/2 teaspoon more cider vinegar.
Storage: refrigerate the soup base 3 to 4 days or freeze it up to 3 months; add bread and cheese only when serving.