Preheat oven to 375 degrees. Line a baking sheet with parchment paper and set it aside.
If you are using homemade dough, roll it into an 18-inch circle on a floured surface. Place it on your prepared baking sheet.
In a small bowl, mix the sugar and cornstarch. In a medium-sized bowl, combine the raspberries and sugar mixture. Gently fold this with a rubber spatula so you don't break up the berries too much.
Pile the raspberries in the middle of the pie dough, leaving a 2-inch space all the way around.
Fold the edges up and over the raspberry filling, this should leave about a 4-inch circle on the inside exposed with the berries.
Mix the egg and milk together and brush around the edges of the galette. Sprinkle with the sugar and place it in the oven.
Bake until the pie dough is golden brown on top and the middle is bubbly. Mine took about 28 minutes. Plan on 25-40 minutes, depending on whether your berries were frozen or not and your oven.
Remove it from the oven. It can be served hot or you can wait until it is cooled.