"Every holiday soiree needs an easy and elegant appetizer to keep guests satisfied until the main event. This recipe from Chef Justine Kelly is just the thing: simple, pretty and delicious, with caramelized onions, apples and grated Gruyère cheese atop store-bought puff pastry...."
INGREDIENTS
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2 Tbs. vegetable oil
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1 1/2 lb. yellow onions, halved and thinly sliced
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1 tsp. kosher salt, plus more, to taste
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2 Tbs. unsalted butter
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1 lb. tart apples, peeled and sliced 1/8 inch thick
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1 tsp. sugar
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1 1/2 oz. aged Gruyère cheese, coarsely grated
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2 Tbs. minced fresh chives
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1 tsp. chopped fresh thyme
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1/8 tsp. freshly ground pepper, plus more, to taste
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1 package (1 lb.) frozen puff pastry dough, thawed
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1 egg, beaten with 2 Tbs. water