Add the flour, sugar and salt to a food processor and pulse just until combined. Add the cold butter pieces into the food processor and pulse until small coarse crumbs remain.In a small bowl, whisk together the egg, vinegar and water. Sprinkle the water/egg mixture over the flour and pulse again until the dough comes together.
Remove the dough with your hands and wrap it in plastic wrap. Refrigerate the dough for 30 minutes.
After 30 minutes, preheat the oven to 400 degrees F.
Add the sliced pears to a bowl. Sprinkle with cornstarch, sugar ginger and salt. Toss the pears well and let them sit for 10 minutes.
Remove the crust from the fridge. Roll it into a “rustic” shape – no shape necessary really – until it is about 1/4 inch thick. Place the dough on a parchment-lined baking sheet. Layer the pears in the center leaving a 2 to 3-inch border of crust. Once the pears are layered, fold the crust over top of the pears. Brush the crust with the beaten egg wash. Sprinkle with the coarse sugar, then press on the crumbled blue cheese.
Bake the galette until the pears are bubbling and the crust is golden, about 40 to 45 minutes. Before serving, sprinkle with more crumbled blue cheese, especially in the center with the pears where it will melt.