Preheat the oven to 425 degrees Fahrenheit
Line a cookie sheet with parchment paper.
(is your puff pastry thawed out?)
In a medium bowl... combine the flour, sugar and bit of salt
Cut the butter into pieces and add it to the flour/sugar
Use a fork or a pastry cutter (or your fingers) to cut the butter into the flour until there are no big butter lumps. It should look a bit like wet sand, and will clump if you squeeze it.
Lay the Puff Pastry Sheet on the parchment paper covered cookie pan.
Use a pastry brush to wet the puff pastry a LITTLE BIT
Now use your hand to sprinkle the streusel over the puff pastry sheet. You can clump it a bit with your fist. Make big and small lumps, and cover the sheet as evenly as you can.
Put the Prasselkuchen into the oven for 17 minutes... then check. Is it browning nicely? You can take it out.
Otherwise leave in for a few more minutes (no more than 20)
Let cool for a few minutes on a rack while you mix the glaze.
Put the Powdered Sugar in a bowl, strain in the lemon juice (you just don't want seeds).
Stir until smooth. It will be a little looser than a regular glaze, and that's good. Don't skimp on the lemon, you need the tart to balance the sweet.
Drizzle the glaze over the warm pastry. It won't cover it completely, but try to get it everywhere.
When you are ready, cut into pieces.
Serve