To make the pastry, put the flour, polenta, salt and
sugar in a food processor fitted with the dough blade. Add the butter and pulse
about 10 times to start breaking up the butter, then process for about 15 seconds,
until the mixture resembles breadcrumbs and there are pea-sized lumps of butter.
Add the water and greek yogurt, pulsing
until the mixture just starts to clump together, adding an additional
tablespoon of water at a time if needed. As long as it sticks together when you
use your hands to squeeze it into a ball, you're set. You don’t want to overprocess, there should still be flecks of butter showing.
Flatten the dough and wrap in cling
film. Chill in the fridge for about an hour until just firm.
Preheat the oven to 350 degrees F. While
the dough is chilling, make the filling by putting the rhubarb, sugar, lemon zest and cornstarch in a roasting dish and mixing. Bake for about for about 15 minutes until the rhubarb has softened and shrunk a bit. Remove and mash the
raspberries so that the color fully distributes. Taste and stir in a little more sugar if needed. Set aside.
When the dough has chilled, roll it out
on a well-floured surface or parchment paper until it’s about 3mm thick. Chill
the flattened dough for another half an hour or so.
Remove the dough from the fridge and
using pizza cutter, cut out squares about 2 ½ inches wide by 4 inches long.
Place a spoonful of the filling on one square, brush the edges with beaten egg
and place another square over top, pinching the dough around the jam and sides
to seal. Cut a vent in each pie and freeze for about 40 minutes, until firm.
Return oven to 350 degrees. Remove pies
from freezer and brush each with the beaten egg, then bake on a sheet pan lined
with parchment paper for 25 minutes, or until golden brown.