Preheat your oven to 400 degrees F. Line a rimmed baking sheet with parchment paper. If you are using refrigerated store-bought dough, take it out of the refrigerator 10 minutes before rolling so it is pliable but still cold.

In a large bowl, combine the sliced pears, brown sugar, cinnamon, nutmeg, cornstarch, lemon juice, and vanilla. Toss gently until every slice is coated. Set aside while you roll the dough.

Unroll or roll the pie dough on a lightly floured surface into a rough circle about 12 inches across. It does not need to be perfectly round, that is the whole point of a galette. Transfer the dough circle to the prepared parchment-lined baking sheet.

Pile the pear mixture into the center of the dough, leaving a 2-inch border all the way around. Use a slotted spoon if there is a lot of juice in the bowl, leaving most of that liquid behind so the crust does not get soggy. Dot the top of the pears with the small pieces of butter.

Fold the border of the dough up and over the outer edge of the pears, pleating as you go. The dough will overlap in places and gap slightly in others. Press the pleats lightly so they hold. It should look like a rustic open tart.

Brush the folded crust edges generously with the egg wash. Sprinkle the coarse sugar evenly over the brushed crust. This is the step that gives the finished galette that sparkly, bakery-style look.

Bake for 35 to 40 minutes until the crust is deep golden brown and the pear filling is visibly bubbling in the center. If the crust edges are browning faster than the filling is cooking, lay a small piece of foil loosely over the browned spots.

Remove from the oven and let the galette rest on the pan for at least 15 minutes before slicing. The filling is very hot and will thicken slightly as it cools. Serve warm or at room temperature, with vanilla ice cream or a spoonful of creme fraiche.