2 tablespoons unsalted butter, plus softened butter, for spreading
• 3 large onions (about 2 pounds), halved lengthwise and thinly sliced crosswise
• Sea salt
• 2 tablespoons dry sherry
• 1 quart Rich Beef Stock (I make a day ahead of time or you can use bone broth)
• Freshly ground pepper
• Four 1/2-inch-thick slices sourdough bread, cut into 4-inch rounds
• 1 bouquet garni, made with 1 bay leaf, 1 thyme sprig, 2 juniper berries and 2 flat-leaf parsley sprigs, tied in cheesecloth
• 2 cups shredded Gruyère cheese (about 6 ounces)
Melt the butter in a large enameled cast-iron casserole. Add the onions and a pinch of salt, cover and cook over moderate heat, stirring once or twice, until the onions soften, about 10 minutes. Uncover and cook over moderate heat, stirring frequently, until the onions are lightly browned, about 40 minutes.
Stir in the sherry. Add the stock and bouquet garni and bring to a boil. Cover and simmer over low heat until the soup has a deep flavor, about 30 minutes. Discard the bouquet garni and season the soup with salt and pepper.
Preheat the oven to 350°. Butter the bread on both sides and place on a baking sheet. Toast the bread for 15 minutes, turning the slices halfway through, until golden and crisp but not dried out. Raise the oven temperature to 425°.
Bring the soup to a simmer, ladle it into 4 deep ovenproof bowls and sprinkle with half of the cheese. Place a crouton in each bowl and sprinkle on the remaining cheese. Bake the bowls of soup on a baking sheet in the middle of the oven for 10 minutes, or until the cheese is bubbling. Serve hot.
Rich Beef Stock
• 1 teaspoon vegetable oil
• 5 pounds meaty beef shanks, cut into 2-inch pieces
• 2 large carrots, cut into 2-inch lengths
• 2 celery ribs, cut into 2-inch lengths
• 1 large onion, quartered
• 4 quarts water
• Sea salt
Preheat the oven to 450°. Heat the oil in a large roasting pan set over 2 burners. Add the beef shanks and cook over moderate heat until sizzling and lightly browned on 1 side, about 5 minutes. Transfer the pan to the oven and roast for 45 minutes, or until the meat and bones are browned. Add the carrots, celery and onion and roast for about 30 minutes longer, or until the vegetables are lightly browned.
Scrape the meat, bones and vegetables into a stockpot. Set the roasting pan over high heat and add 1 cup of the water. Cook, scraping up the browned bits, until the pan is clean. Pour the pan juices into the stockpot along with the remaining 3 cups and 3 quarts of water and simmer over moderately high heat for 30 minutes, skimming occasionally. Reduce the heat to moderately low, cover partially and simmer until the stock is richly flavored and reduced to 2 quarts, about 4 hours. Season with salt.
Strain the stock through a fine sieve set over a heatproof bowl. Refrigerate until cold, scrape off the fat and discard. Before using, boil the stock until reduced to 6 cups.