Ingredients
3 pounds yellow onions, peeled, sliced, and cut into quarter-moons
2 tablespoons butter, melted
2 tablespoons olive oil
2 teaspoons salt
Freshly ground black pepper
10 cups reduced-sodium beef broth
2 tablespoons balsamic vinegar
4 to 6 toasted baguette slices per bowl
1/3 cup grated mozzarella per bowl
Instructions
Place the onions in a 5-quart or larger slow cooker. Stir in the butter, oil, salt, and a generous amount of pepper. Place in crock pot and cook on the low until the onions should be dark golden-brown and soft, 12 hours. Add the broth and vinegar. Stir in the broth and vinegar. Cover and continue cooking on the low setting for 6 to 8 hours. Longer cooking will only intensify the flavors. Taste and season with more salt and pepper if needed. Arrange a rack in the upper third of the oven and heat to 350 degrees. Ladle the soup and onions into oven-safe soup bowls and place the bowls on a rimmed baking sheet. Top each bowl with a slice of toast and a generous quantity of shredded mozzarella cheese. Bake until the cheese is completely melted, 20 to 30 minutes. Turn the oven to broil. Broil until the cheese is bubbling and browned, 2 to 3 minutes. Remove from the oven and let cool for a few minutes.
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