Add all-purpose flour, sugar, and baking powder to a large bowl. Add cold butter cubes. Using your fingertips, rub the butter into the flour mixture until there are no lumps or chunks of butter left. Feel free to use a blender or food processor to make the dough. Add the lemon zest egg yolk and liquid (wine, milk or water) to the flour mixture and knead until it all comes together, for about 30 seconds. Don't overwork it. It should remain crumbly. Divide the dough in half.
roll out the dough
Dust your working surface with flour. Place a sheet of parchment paper over the dusted surface. Place one half of the dough in the center of the parchment paper and cover with another sheet of parchment paper. Using a rolling pin, roll the dough to approximately 0.5 cm (1/4-inch) thickness. Repeat the process with the remaining half of the dough. Place each dough on a separate tray or baking sheet (lined with parchment paper) and place it in the fridge for 1 hour or in a freezer for 30 minutes.
transfer to a baking sheet
Remove the dough from the fridge (or freezer) and roll it into 2 - 3 mm (1/8 inch) thickness. Transfer half of the dough to a baking sheet (20 cm x 35 cm / 9x13 inch). Prick the dough with a fork and trim the dough so there is about 2 cm / 1 inch overlapping the edge of your baking sheet. Tuck the dough under and onto itself, to give it a neat folded edge.
Clean and peel pears. Cut in quarters, remove the core and slice them thinly. Add the sliced pears to a large bowl. Add brown sugar, cornstarch and ground walnuts. Stir to combine and set aside until needed.
Blueberry Pear Slab Pie (Lattice Crusted)
Spread high-quality blueberry jam over the dough in the baking sheet. Evenly spread the pear filling over the blueberry jam. Cut 2 - 2,5 cm (1 inch) stripes from the rest of the dough and create a desirable lattice finish on top of the filling. Using a brush, gently brush the lattice and the edges of the pie with the egg wash. Sprinkle with brown sugar. Place in the preheated oven. Bake for about 15 - 20 minutes at 200 °C / 390 °F, then lower the heat and bake for another 40 - 45 minutes at 170 °C / 345 °F.
Remove the Blueberry Pear Slab Pie from the oven and let it cool slightly on the baking sheet. Cut on even slices (you will get around 20 slices) and serve. Keep chilled in an airtight container for up to 3 days