Creating the Mixed Berry Tart is enjoyable and straightforward. Follow these steps for a successful outcome:

1. Preheat the Oven: Begin by preheating your oven to 350°F (175°C).

2. Make the Crust:
– In a mixing bowl, combine the flour and sugar.
– Add the cold, cubed butter and mix until the mixture resembles coarse crumbs.
– Stir in the egg yolk and add ice water, one tablespoon at a time, until the dough comes together.
– Form the dough into a disk, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

3. Roll Out the Dough:
– On a lightly floured surface, roll out the chilled dough to fit a 9-inch tart pan.
– Carefully transfer the dough to the tart pan, pressing it into the edges. Trim any excess.

4. Bake the Crust:
– Prick the bottom of the crust with a fork to prevent bubbling.
– Bake in the preheated oven for 15-20 minutes or until golden brown.
– Remove from the oven and allow it to cool completely.

5. Prepare the Filling:
– In a mixing bowl, combine the mascarpone cheese, heavy cream, powdered sugar, vanilla extract, and lemon zest.
– Whip the mixture until it is smooth and creamy.

6. Assemble the Tart:
– Spread the mascarpone filling evenly over the cooled crust.
– Arrange the fresh berries on top of the filling in a decorative pattern.

7. Chill the Tart:
– Place the assembled tart in the refrigerator for at least 30 minutes to set.

8. Garnish:
– Before serving, garnish with fresh mint leaves if desired.