Heat the oven. Preheat to 425°F (220°C). Line a baking sheet with parchment paper, not foil, so the bottom stays crisp. On a dark nonstick sheet, use 400°F (205°C).

Prep the apples. Peel, core and halve 3 medium apples, then slice them 1/8 inch thin using a mandoline slicer or a knife. Toss with 1 tbsp lemon juice.

Make the egg wash. Beat 1 large egg in a small bowl.

Stack the pastry. Lay 1 sheet of puff pastry on the parchment and brush off any flour with a dry pastry brush. Brush with egg wash. Brush the flour off the underside of the second sheet, set it on top and press so they stick. If the pastry softens, chill it for 10 minutes.

Score and dock. Create the tart border by cutting a line 1/2 inch in from the edges, halfway through the pastry. Prick the center all over with a fork so it stays flat under the apples. Brush the top with egg wash, keeping it off the cut edges so the border can rise.

Add the apples. Arrange the slices in overlapping rows inside the border. Mix 1/3 cup granulated sugar and 1/2 tsp ground cinnamon and sprinkle evenly over the apples.

Bake. Bake on the lower-middle rack until the edges are puffed and deep golden and a knife slides easily into the apples, about 25 to 30 minutes. Cover the edges with parchment strips if they brown too fast.

Glaze. Microwave 2 tbsp apricot jam for 15 to 20 seconds, push it through a fine-mesh sieve and brush over the hot apples.

Cool and serve. Cool for at least 15 minutes. Dust with 1 tsp powdered sugar and cut into 9 squares.