Defrost the pastry squares according to package instructions. Drain the peach halves well. Place a peach half in the center of each square and brush the edges of the dough with egg yolk.
Place the squares on a baking sheet lined with parchment paper. Bake in a heated oven at 200 °C for 15 to 20 minutes, then cool.
Mix the powdered sugar with the juice of one lemon until smooth and pour an “egg white mirror” around each peach half.