Set the oven to 400 degrees Fahrenheit. Lay your pie crust dough on a piece of parchment paper and use a rolling pin to roll it into a 12” diameter circle. Transfer the parchment paper with the dough circle onto a rimmed baking sheet.
In a large mixing bowl, add the brown sugar, cornstarch, cinnamon, and salt and stir to combine. Add the lemon juice and stir until well mixed. Add the sliced pears and stir until they are coated in the brown sugar mixture.
Arrange the pears on top of the pie crust dough, leaving a 2-inch border around the outside. I overlapped the slices in a circle around the outside and then filled in the center with a few sliced before adding another circle on top, overlapping the original circle about halfway in. Then I did a third circle overlapping that and arranged the final pieces in the center. You can take this concentric circle approach or just spoon the pears into the center of the dough and then spread them out evenly.
Fold the outside edge of the pie crust dough up and over the edges of the filling. Drop little bits of the butter over top of the pear filling.
Crack the egg into a small dish and add the tablespoon of water. Use a fork to beat them together until combined into an egg wash. Use a pastry brush to paint the egg wash over the folded crust dough (you will not use anywhere near all of it, just discard whatever remains). Sprinkle the two teaspoons of sugar all over the egg washed crust and the pears in the center of the galette.
Place the baking sheet in the heated oven and bake for 35-40 minutes until the crust is golden and the pears are hot and bubbly. Transfer the parchment with the galette onto a cooling rack and let it cool for at least 20 minutes before slicing.