Creating a Mixed Berry Tart is straightforward if you follow these clear steps:

1. Prepare the Tart Crust: In a medium bowl, whisk together the flour, powdered sugar, and salt.
2. Incorporate Butter: Add the chilled and cubed butter to the flour mixture. Use a pastry cutter or your fingers to mix until it resembles coarse crumbs.
3. Add Egg Yolk: Stir in the egg yolk and cold water. Mix until the dough comes together; do not overwork it.
4. Chill the Dough: Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
5. Preheat Oven: Preheat your oven to 375°F (190°C) while the dough chills.
6. Roll Out the Dough: On a floured surface, roll out the dough into a circle, about 1/8 inch thick.
7. Transfer to Tart Pan: Gently place the dough in a 9-inch tart pan. Press it into the bottom and sides. Trim excess dough.
8. Blind Bake the Crust: Line the crust with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes. Remove the weights and bake for an additional 10 minutes until golden. Let it cool slightly.
9. Prepare the Berry Filling: In a mixing bowl, combine the mixed berries with granulated sugar, cornstarch, lemon juice, and vanilla extract. Gently toss to coat the berries evenly.
10. Assemble the Tart: Pour the berry mixture into the pre-baked tart crust, spreading it evenly.
11. Make Egg Wash: Beat the egg and brush it over the crust’s edges for a golden finish.
12. Bake the Tart: Bake the assembled tart for 25-30 minutes, or until the berries bubble and the crust is golden brown.
13. Cool Down: Remove the tart from the oven and let it cool on a wire rack for at least 15-20 minutes before slicing.
14. Garnish: Optional—add fresh mint leaves on top for a pop of color and flavor before serving.

These simple steps will guide you in producing a stunning and delicious Mixed Berry Tart that will wow your guests!