Preheat oven to 400 F: As you preheat, notice the changing hum of the oven as it approaches 400 F , the air warming and the promise of golden pastry. Preheating to the correct temperature ensures the puff pastry gets immediate oven spring, creating distinct flaky layers. If you skip full preheating the pastry can bake slowly and become greasy rather than crisp, so wait until the oven indicator is steady.
On a parchment lined baking sheet, roll out the puff pastry dough to a large rectangle.: When you roll the chilled puff pastry , the sound is faint and flour dust whispers under the pin as layers smooth. Roll gently from the center to the edges to keep the laminated layers even. If you press too hard or roll unevenly, the layers can compress and the final flakiness will suffer, so aim for an even rectangle with consistent thickness.
In a small bowl, whisk together the egg and the water.: The egg wash should shimmer slightly as you whisk the egg with water , breaking down the yolk into a smooth, glossy liquid. This dilution creates a thin coat that browns evenly. If you overbeat and incorporate too much air the wash may bubble and create tiny pinprick spots of color, so whisk just until uniform.
Use a knife to score a border 3/4 of an inch inside from the edge of the puff pastry. Use a fork to pierce the inside of the tart several times.: Scoring a border creates a raised frame, and the fork pricks let steam escape from the center so it stays flat. You will feel the pastry give under the knife and the scored line guides how the pastry puffs. Be careful not to cut all the way through when scoring; a shallow cut is enough. If you pierce too deep you will compromise the rise along the edge.
Brush the pastry with the egg wash along the border. Sprinkle with sparkling sugar if desired.: Brushing the border with the egg wash adds a glossy sheen that transforms the baked edge. The brush should glide smoothly, leaving a thin film that reflects light. Sprinkle sparkling sugar sparingly for a delicate crunch. Avoid pooling the wash in crevices, because it can overbake and darken unevenly.
Bake for 20 minutes until golden brown and cooked. Set aside to cool.: During baking you will hear a gentle whisper as the layers separate and the pastry puffs. The aroma shifts to a warm, buttery perfume and the color deepens to golden brown. Let it rest on the sheet so residual heat finishes the bake. Removing it too early can cause the layers to collapse, so allow it to cool and set for a firmer surface to top.
While the puff pastry is cooking, prepare the topping. In a large bowl, stir together the whipped cream, 1/2 cup white sugar, and mascarpone cheese.: As you fold the whipped cream into the mascarpone with the measured sugar, the texture should turn glossy and billowy. The mixing creates a silky cream that supports the fruit. Avoid overmixing, which can break down the whipped cream and make the filling runny; fold gently until homogeneous and smooth.
In a separate bowl, mix together the strawberry slices, 1/4 cup white sugar, and strawberry jam.: Toss the sliced strawberries with sugar and strawberry jam until they glisten and release a little juice. That maceration intensifies flavor and creates a syrupy glaze that helps the berries adhere. Be mindful that overly soft berries will become too liquid; use ripe but firm fruit to keep texture.
Once puff pastry is cool, top with the cream and then the strawberries. Keep refrigerated until ready to slice and serve.: When assembling on a cooled crust, the filling will stay pristine and the layers remain separate. Spread the cream evenly so its pale, satiny surface contrasts with the bright berry topping. Refrigerating lets flavors marry, but if you chill too long the pastry can soften slightly from the fruit juices, so slice within a reasonable window after assembly for the best texture.
Top with vanilla ice cream. Enjoy!: A scoop of ice cream provides a cold, creamy counterpoint if serving warm slices. The melting ice cream creates tiny rivulets of richness against the fruit, elevating each bite. If you add it too early the ice cream will melt excessively; add just before serving to preserve its texture.