INGREDIENTS
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3 tablespoons unsalted butter
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15 cups sliced yellow onion (about 4 pounds)
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3/4 teaspoon black pepper
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1 Honeycrisp or Pink Lady apple, peeled, quartered, and cut into julienne strips
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3 thyme sprigs
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2 bay leaves
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1/2 cup Madeira wine or dry sherry
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6 cups lower-sodium beef broth
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1/2 cup apple cider
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1 tablespoon sherry vinegar
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10 (1/2-ounce) slices sourdough bread, cut into 1-inch cubes
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2 cups (8 ounces) grated Gruyère or Swiss cheese
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Thyme leaves (optional)