Divide the pie dough into two flattened discs; wrap in plastic and chill.
Preheat the oven to 375 degrees F. Line two cookie sheets with parchment.
Working one disc at a time, roll out the dough into a 1/8-inch-thick rectangle on a floured work surface. Cut into 9 even rectangles, then transfer to the prepared cookie sheets and chill.
Meanwhile, in a heavy-bottomed saucepot, add the pears, apple juice, lemon juice and sugar. Cook over medium heat until the pears begin to soften and the liquid thickens, 20 minutes. Set aside and let cool.
When the pear filling has cooled, remove one of the cookie sheets of portioned dough to a work surface. Whisk the egg and brush it evenly on the surface of the rectangles. Add 1 tablespoon of the pear filling to each rectangle. Remove the second cookie sheet from the refrigerator and place the remaining rectangles on top of the pear filling, pressing the edges with your fingers. With a fork, go over edges to seal. Poke holes in the top to allow steam to exit. Place the assembled tarts back in the refrigerator for 30 minutes.
Bake for 16 minutes, until golden brown, rotating every 5 minutes for even cooking. Let cool for 5 to 7 minutes.
In a small bowl, mix together the powdered sugar, milk, lemon zest and vanilla. Ice each tart and sprinkle with the poppy seeds.