In a food processor, blend the flour, cornstarch, sugar and salt.
Add the diced, unsalted butter and pulse until the texture is coarse, resembling the shape of a pea-sized crumb.
Add 4 tablespoons of buttermilk until just combined. If the mixture seems dry, add up to 2 more tablespoons of buttermilk. Add the first spoonful first, if it is still dry, then add the second spoonful of buttermilk.
Transfer the dough to a sheet of plastic wrap. Fold the corners of the plastic wrap so that the dough is shaped like a square. Wrap and refrigerate for at least 1 hour.
Filling
Mix the brown sugar, cornstarch, lemon juice, and salt together in a bowl.
Peel the pears, core and slice them thinly. Make the slices as even as possible. (See notes)
Toss the pears in the sauce until they are evenly coated. Set aside.
Make the Galette
Preheat the oven to 400°F. Line the baking sheet to be used with parchment paper or silicone mat. Prepare an egg wash by whisking together the egg and milk in a small bowl.
Take the dough out of the refrigerator. Dust the surface where you will roll the dough with flour.
Roll the dough out into a 12-inch circle.
Leaving 2 to 3 inches from the edge of the dough, arrange the pears in a circle on the outer edge of the dough.
Add additional rows in a concentric circle. Make sure you don’t stack the pears too high.
Lift and press the edges of the dough up and over the fruit. Make sure the edges can keep the pears from falling apart out of the dough.
Transfer the galette to the prepared baking sheet and refrigerate for 10 minutes.
Brush the edges of the galette with the egg wash and sprinkle with turbinado sugar.
Bake the dough until the filling is tender and the crust is golden brown, about 30-45 minutes.
Transfer the galette to a wire rack to cool.
Glaze and Serve
While the galette is cooling, add the preserves to a small saucepan. Heat over low heat until the mixture becomes liquid. If there are pieces of fruit in the preservative, strain them through a fine sieve until there are no more chunks.
Use a pastry brush to gently and evenly glaze the top of the galette with the preserves.
Let the glaze set for 10-15 minutes then serve with a scoop of vanilla ice cream. Enjoy!