Thaw the puff pastry.

Leave the puff pastry at room temperature for 30 to 40 minutes, or until it is soft enough to unfold without cracking. You can also microwave it by wrapping it in paper towels and heating it for 15 seconds on each side. If needed, microwave for a few additional seconds.

Cut the pastry.

Cut the pastry sheet into 6 rectangles, approximately 4.5 x 3.5 inches. Slight variations in size are fine.

Score the pastry.

Place the rectangles on a parchment-lined or silicone-lined baking sheet. Score the top of each rectangle with a knife without cutting completely through the pastry. This will create an area that can be pressed down after baking to hold the filling.

Add the egg wash and sugar.

Whisk the egg and milk together. Lightly brush the mixture over each pastry rectangle, then sprinkle with coarse sugar.

Bake.

Bake at 400°F (200°C) for 12 to 15 minutes, or until the pastry is puffed and golden brown. If your oven runs hot or you are using convection, bake at approximately 375°F (190°C). Baking time can vary between ovens.

Cool completely.

Transfer the baked pastries to a cooling rack and let them cool completely before adding the fillings.

Make the mascarpone frosting.

Beat the cold whipping cream and powdered sugar until soft peaks form. Add the lemon zest, mascarpone cheese, and vanilla. Continue whipping until the mixture is smooth and creamy. Avoid overwhipping.

Make the lemon curd.

Beat the room-temperature butter, granulated sugar, and lemon zest in an electric mixer at medium speed until light and fluffy. Add the eggs one at a time, allowing each egg to become fully incorporated before adding the next.

Cook the curd.

Add the salt and lemon juice and mix on low speed. The mixture may look separated or curdled at this stage, which is normal. Pour it into a small, heavy-bottomed saucepan and cook over medium-low heat, stirring constantly.

Finish the lemon curd.

Continue stirring until the curd becomes very thick and creamy. Do not allow it to boil. When it reaches approximately 170°F, remove it from the heat and pour it into a heatproof bowl. Refrigerate until completely chilled.

Assemble the pastries.

Use a knife to cut around the scored area on top of each cooled pastry. Gently press down the center to create a space for the filling. Pipe or spoon in the mascarpone frosting, then add lemon curd.

Add the finishing touches.

Top each pastry with mixed berries and lightly dust with powdered sugar. Serve immediately for the best texture.