Caramelized Onion and Apple Tarts with Gruyere and Thyme

INGREDIENTS

• 2 Tbs. vegetable oil
• 1 1/2 lb. yellow onions, halved and thinly sliced
• 1 tsp. kosher salt, plus more, to taste
• 2 Tbs. unsalted butter
• 1 lb. tart apples, peeled and sliced 1/8 inch thick
• 1 tsp. sugar
• 1 1/2 oz. aged Gruyère cheese, coarsely grated
• 2 Tbs. minced fresh chives
• 1 tsp. chopped fresh thyme
• 1/8 tsp. freshly ground pepper, plus more, to taste
• 1 package (1 lb.) frozen puff pastry dough, thawed
• 1 egg, beaten with 2 Tbs. water