"Plus a tip for faster-cooking caramelized onions...."
INGREDIENTS
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3 tablespoons unsalted butter
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2 pounds yellow onions, thinly sliced
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1/8 teaspoon baking soda
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Kosher salt and freshly ground black pepper
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1 tablespoon olive oil
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3 pounds boneless, skinless chicken thighs
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2 cups low-sodium chicken or beef broth, divided
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2 tablespoons Dijon mustard
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1 tablespoon balsamic vinegar
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2 to 4 cloves garlic, thinly sliced
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2 fresh thyme sprigs
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1 (4-inch) fresh rosemary sprig
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1 1/2 cups shredded Gruyère cheese (about 5 ounces)
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Coarsely chopped fresh parsley, for garnish (optional)