Preheat your oven to 400°F and line a baking sheet with parchment paper. On a lightly floured surface, roll the puff pastry into an even rectangle, then cut it in half. Work with one half at a time, keeping the other half in the refrigerator until you are ready to use it. Chilling the unused portion makes it easier to roll the dough thin and keeps the butter layers cold for the best flaky texture.
Roll the dough into 4-inch by 4-inch squares, about ⅛ inch thick. From one half of the pastry, you should be able to cut 8 to 9 squares, depending on how evenly the dough is rolled.
Spoon about 1 tablespoon of strawberry jam into the center of each square, being careful not to overfill, as the jam can run during baking. Fold one corner of the pastry over the jam. Lightly brush the opposite corner with egg wash, then bring it over the folded corner, pressing gently so it adheres and forms a neat overlap. This creates the classic folded Danish pastry look while leaving the filling visible.
Transfer the pastries to the prepared baking sheet and brush the pastries lightly with egg wash.
Bake for 18 to 22 minutes, or until the pastries are puffed and golden brown. Allow the strawberry danishes to cool slightly at room temperature. Whisk powdered sugar with a small amount of milk or lemon juice until smooth, then drizzle over the pastries before serving.