Preheat oven to 400 degrees F. Line a baking sheet with parchment paper; set aside. On a floured surface unroll and fold the pastry sheet in quarters then roll out gently into a 12x10-inch rectangle. Trim edges. Carefully transfer pastry sheet to prepared baking sheet. Brush pastry with a mixture of egg yolk and 2 teaspoons water and sprinkle with rosemary. Arrange chard on top, pressing in lightly. Sprinkle with cheese. Bake 15 minutes or until golden. Remove; cool completely. Sprinkle with sea salt before serving.