What could be better for a summertime party than individual tarts filled with the best stone fruit of the season? When it comes to making truly good tarts, you won't want to cut corners, which is why we make everything from scratch.
Spread the shortening on a tray and place in the freezer for about 20-30 minutes. Combine the vodka and water and place in the freezer until well chilled, about 10-20 minutes.
Combine the flour, salt, and sugar in the bowl of an electric mixer. Add the butter and mix until the butter is cut into about ¼-inch pieces. Cut the frozen shortening into pieces, add, and mix until broken up. (Be careful not to mix too long; you don't want the dough to start coming together before adding the liquid.)
Add the vodka-water mixture and mix until the dough is formed and combined (it's OK to still have small pieces of butter). Wrap in plastic wrap and chill the dough overnight in the refrigerator. The next day, lightly flour a work surface and rolling pin and roll out very thin, about 1/8-inch thick.
Return to the refrigerator to chill. Using a 3-inch round cutter, punch out circles, and place on a baking sheet in the freezer. (The dough will be very soft so it is important to keep it in the freezer.)
Pit and slice 1 yellow nectarine and 1 white nectarine. Place 2 halves of yellow nectarine on top of each round. Then, make a new layer with alternating yellow and white slices of nectarine. Cut more as needed.
Preheat the oven to 350 degrees.
In a small bowl, mix together the heavy cream and egg yolk until combined to make the egg wash. Brush the edges of each circle with the egg wash. Make about 5 folds around each tart, and press to form around the fruit.
After all the tarts are folded, egg wash the outside and press each fold into the tarts so they don't unfold during baking. Sprinkle raw or turbinado sugar on the outside of each tart and generously sprinkle granulated sugar on the fruit slices in each tart.
Bake until the crusts are golden brown and the fruit is tender, about 12-15 minutes, rotating halfway through. Place on a cookie rack to cool so bottoms don't get soggy.