Hearty Lentil French Onion Soup With Melty Gruyère

Ingredients

For 4 servings

• •4 c. à soupe (57 g) unsalted butter, plus more for bread
• •2 pounds (900 g) yellow onions, thinly sliced (about 7 cups; see notes)
• •1 1/2 teaspoons Diamond Crystal kosher salt, plus more to taste; for table salt, use half as much by volume
• •1 /2 teaspoon freshly ground black pepper
• •1 /4 cup (120 ml) dry sherry
• •7 tasse (1.65 L) homemade beef stock or homemade or store-bought low-sodium chicken stock
• •1 tasse dried French le Puy lentils (7 ounces; 200 g), picked over for stones, rinsed, and drained
• •2 fresh thyme sprigs, plus thyme leaves for garnish
• •1 dried bay leaf
• •1 c. à café (5 ml) apple cider vinegar
• •8 (1/2-inch-thick) French bread slices, trimmed to fit bowl size, and toasted until crisp
• •1 medium clove garlic
• •8 ounces (226 g) Gruyère cheese, grated (about 2 cups)

Method

1. 1.In a large Dutch oven, melt butter over medium-high heat until foaming. Add onions, salt, and pepper; cook, stirring occasionally, until onions are softened, 6 to 8 minutes.
2. 2.Reduce heat to medium-low and cook, stirring frequently and scraping bottom of pot as needed, until onions are sweet and deeply caramelized, 45 minutes to 1 hour. If browned onion juices on bottom of pot threaten to burn, add 1 tablespoon (15 ml) water, scrape up browned bits, and continue cooking.
3. 3.Add sherry, scraping up any browned bits, and cook over medium until alcohol is mostly evaporated, 2 to 3 minutes.
4. 4.Add stock or broth, lentils, thyme sprigs, and bay leaf. Bring to a boil over medium-high heat, then reduce heat to medium-low, cover, and simmer, stirring occasionally, until lentils are tender but hold their shape, 25 to 30 minutes.
5. 5.Remove and discard thyme sprigs and bay leaf. Stir in apple cider vinegar. Season with additional salt and pepper to taste.
6. 6.Preheat broiler and adjust oven rack to about 6 inches from heat source. Lightly butter toasted French bread slices, and rub with garlic clove until fragrant.
7. 7.Ladle soup evenly into 4 broiler-safe bowls. Top each bowl with 2 slices of bread, overlapping slightly. Sprinkle each bowl evenly with 1/2 cup (2 ounces) Gruyère cheese.
8. 8.Place bowls on a rimmed baking sheet. Broil until cheese is melted and browned in spots, about 3 to 4 minutes. Garnish with thyme leaves, and serve immediately.