I preheat my oven to 400°F and line a baking sheet with parchment paper. Then I roll the thawed puff pastry out gently into a neat rectangle, keeping the thickness even. This step sets the shape of the tart, so I take my time here.
Using a sharp knife, I score a border about ¾ inch in from the edges, being careful not to cut all the way through. Then I use a fork to prick the center several times. This lets the middle stay flatter while the border puffs up into a beautiful edge.
I whisk together the egg and water in a small bowl until fully combined. Then I brush this mixture along the scored border only, which helps it turn deep golden in the oven. If I'm feeling fancy, I sprinkle a little sparkling sugar on top for extra shine.
Into the oven it goes for about 20 minutes, until the border is puffed and golden brown. I keep an eye on it toward the end since ovens vary. Once it's done, I set the sheet aside and let the pastry cool completely before adding any topping.
While the pastry cools, I combine the whipped cream, half of the sugar, and the mascarpone cheese in a bowl. I mix until it's smooth and thick, with no lumps left. This creamy layer is what makes the tart feel indulgent without being heavy.
In a separate bowl, I mix the sliced strawberries with the remaining sugar and the strawberry jam. I let this sit for a few minutes so the berries soak up some of that sweetness and turn glossy before I use them.
Once the pastry is fully cool, I spread the mascarpone cream evenly inside the border, then arrange the strawberries on top. I pop the whole tart in the fridge for about 15 minutes so it firms up slightly, which makes slicing much cleaner.