Pulse the flour, cornmeal, sugar, and salt in a food processor to combine. Add the room temperature butter and pulse until the fat completely disappears into the dry ingredients.
Add half of the chilled butter and pulse until the flour mixture resembles pea-sized crumbs, about 18 pulses. Add the rest of the chilled butter and pulse 3 to 4 more times.
Transfer this mixture to a medium-size mixing bowl, and spritz with just enough juice to moisten the surface. Mix with a spatula. Continue spritzing and mixing until a handful of dough, when squeezed, remains compressed — you might not use all the juice. When you've got good adhesion, gather the dough into a round disk, cover with plastic wrap, and refrigerate for 20 minutes so that the flour can absorb the moisture.
Heat a 12-inch nonstick pan over medium heat. Add the pears and toss for 2 minutes. Add the vinegar and continue tossing for another 30 seconds. Sprinkle with the sugar, and cook until the pears have softened, about 2 minutes.
Kill the heat and add the nutmeg, cinnamon, and butter. When the butter has melted, fold in the blueberries. Sprinkle in the 1 teaspoon flour and stir to combine. Cool to room temperature.
Crank the hot box to 400°F.
Place the dough on a floured piece of parchment and roll it out to a 1/4-inch thick disk, rolling the pin from the center out, turning the dough on the paper every few seconds to maintain evenness. Use a pizza cutter to cut the dough into a circle with a uniform edge, then transfer, still on the parchment, to an inverted half-sheet pan.
Place the pound cake in the middle of the dough, leaving about a 3-inch margin all the way around. Spoon the pear filling over the cake cubes and top with the cubed butter. Lift the edges of the dough circle and fold over toward the middle, working clockwise. Brush the crust with the egg wash and sprinkle with sugar.
Bake until the filling bubbles and the crust is golden brown and delicious, 30 to 35 minutes. Remove from the oven, carefully slide off the sheet pan, and cool completely before serving.