Toss the Vanilla Sugar (1/2 cup), All-Purpose Flour (1/3 cup), Ground Cinnamon (to taste), and Lime (1) together in a small bowl and stir to combine.
Place the Black Cherry (2 cup), Plum (1/2 cup), and Frozen Strawberries (1 cup) in a large bowl and, using your hands, toss with the flour/sugar mixture until all of the fruit is coated with the dry ingredients.
To form the crust, Combine All-Purpose Flour (2 1/2 cup) and Vanilla Sugar (1/2 cup) in the bowl of the food processor. Add chilled Unsalted Butter (1 cup) and whiz till breadcrumb like mixture forms.
With a fork, vigorously stir in the lightly beaten Egg (1) until all the dry ingredients are moistened and a dough starts to pull together and form a ball.
Scrape up the dough together, re-flour the surface lightly and work very briefly and quickly until you have a smooth, homogeneous dough. If the dough is a bit too soft or sticky for you, refrigerate it for 10 or 15 minutes until it can be easily rolled out without sticking to your rolling pin.
Cut the dough in two pieces, one half slightly larger than the other. This larger part will be the bottom crust.
On a lightly floured surface, roll out the dough for the bottom crust until you have a circle about an inch (2 cm) wider all around than the bottom of the pie plate. Carefully roll up the dough around the rolling pin and lift and transfer to your pie dish, unrolling into the dish. Gently lift up the dough all around, sliding and pressing the dough down into the dish.
Using a knife or your rolling pin lying flat on top of the pie dish, cut off all excess dough hanging over the edges. Repress the sides of the dough back into the (crimped) sides of the pie dish.
Pour the prepared fruit filling into the pie dish and push the fruit around until evenly distributed and filling any gaps.
Preheat the oven to 375 degrees F (190 degrees C).
On your floured work surface, roll out the remaining dough into a circle the size of your pie plate or slightly larger. Cut into even strips of any width.
Form a lattice over the fruit filling. Using every other strip from the cut circle of dough, space them evenly across the top of the pie, leaving the edges hanging loosely over the edge of the pie.
Trim the edges of the dough strips and press (flour your fingers if they stick to the dough) the strips into the dough of the bottom crust. If you like, use any leftover dough to make decorative shapes for the top (I made hearts). ‘Glue‘ these shapes of dough onto the crust with a bit of Milk (to taste). Gently and lightly brush the crust lattice with milk.
Bake in the preheated oven for 40-50 minutes, until the crust is golden and cooked, and the fruit is bubbling. If any part of the crust begins to brown too fast – the edges of mine began browning well before the center, simply cover with strips of aluminum foil.
Carefully remove from the oven and allow to cool on a cooling rack. The filling will firm up perfectly when cooled to room temperature.
Serve plain, with unsweetened whipped cream or vanilla ice cream.