Preheat oven to 400°F: The moment you set the oven, you ll smell the faint warmth as the heating elements come alive, and that steady rise in heat is essential for the pastry to puff properly. A fully heated oven ensures the layers steam rapidly creating lift and flakiness, while a cold oven yields a dense, greasy result. One mistake to avoid is opening the oven door frequently while the pastry is baking, which lets steam escape and reduces rise.
On a parchment lined baking sheet, roll out the puff pastry dough to a large rectangle.: You ll feel the cool pliability of the dough under your rolling pin, and your motions should be even to prevent thin spots. The parchment helps lift and transfer the tart without tearing. Rolling too aggressively can break the layers, so take gentle, even passes and rotate the dough occasionally to maintain shape.
In a small bowl, whisk together the egg and the water.: The small bowl fills with a glossy amber liquid, and the whisk makes a soft, rhythmic sound as it incorporates the water. This wash creates color and sheen on the pastry border, which cues doneness visually. Avoid adding too much water, which thins the wash and reduces its effectiveness.
Use a knife to scour 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.: The scored border creates a rim that will rise higher than the center, framing the filling, while the fork pricks let steam escape so the center stays flatter for topping. Run the knife lightly to avoid cutting through the pastry entirely. If you press too hard, the rise may be uneven.
Brush the pastry with the egg wash along the border. Sprinkle with sparkling sugar if desired.: The egg wash glides on and gives an immediate gloss, and the sparkling sugar adds a delicate crunch that catches light. Brush in thin, even strokes to prevent the wash from pooling into the center, which could make it soggy. Don t overapply the wash or the pastry can brown too quickly.
Bake for 20 minutes until golden brown and cooked. Set aside to cool.: As the tart bakes you ll hear faint crackles and smell a warm, buttery aroma. Watch for even golden color along the edges, that tells you the layers have set. Remove to a rack to cool so steam escapes and the bottom does not become soggy. A common error is cutting into the tart while it is too hot, which causes the cream and fruit to slide off.
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.: The mixture becomes luxuriously smooth, and the aroma is lightly sweet and dairy forward. Whisk gently until it is homogenous and slightly fluffy, which gives the filling body and allows it to sit on the pastry without collapsing. Avoid overwhipping, which can turn the mixture grainy or separate.
In a separate bowl, mix together the strawberry slices, 1/4 cup white sugar, and strawberry jam.: As you fold the jam and sugar into the strawberries , the fruit releases fragrant juices and becomes glossy. This quick maceration wakes up the berries and deepens their sweetness, producing a jammy glaze that helps the slices adhere. Don t let them sit too long or they will become watery and lose structure.
Once puff pastry is cooled, top with the cream and then the strawberries. Keep refrigerated until ready to slice and serve.: When you spread the creamy layer it should feel cool and velvety under your spoon, creating a stable bed for the fruit. Arrange the strawberry slices with care so each slice looks intentional and each bite has fruit. Chill to set the cream, which makes slicing cleaner. A frequent misstep is applying the fruit while the cream is warm, which causes slipping and messy slices.
