Preheat oven to 350 degrees.
In a mixing bowl combine flour, sugar, and cold butter.
Mix until crumbly.
Add egg yolk.
Add cold water gradually until dough forms.
Chill the dough for 20 to 30 minutes.
Press dough into a tart pan.
Dock the bottom with a fork.
Bake for 18 to 22 minutes until lightly golden.
Cool completely.
Add berries, sugar, and lemon juice to a small saucepan.
Cook until the berries soften.
In a small bowl combine cornstarch and water.
Add the slurry to the berries.
Cook until glossy.
Cool completely.
Add mascarpone cheese, heavy cream, powdered sugar, and vanilla extract to a mixing bowl.
Whip until smooth, light, and soft peaks form.
Spread the mascarpone filling into the cooled tart shell.
Top with the cooled berry topping.
Refrigerate for 30 to 60 minutes.
Garnish with fresh mint.
Serve chilled.