First prepare your potatoes by peeling and grating them. Add the grated potatoes to a cheesecloth and wring out the moisture completely.
Transfer the potatoes to a dry bowl and season with salt and pepper to taste, mixing around to distribute evenly. Add the cornstarch and mix again to incorporate.
Heat a medium sized frying pan over medium to medium-low heat. Once the pan is hot, add the butter and spread it around as it melts. Be careful to not let the butter burn.
Once the butter is hot, spread half of the grated potatoes in a layer covering the entire bottom of the pan. Season it once more with salt and pepper then add the remaining potatoes in another layer.
Cook this first side for about 3-4 minutes until the bottom is golden brown. You can use a spatula to check the color. You should also be able to easily move around the whole galette by moving around the pan.
When you are ready to flip, you can flip it in the pan, however if you need extra help, you can slide the galette onto a plate. Once on the plate, turn the pan over onto the uncooked side, then carefully but quickly flip the plate and pan over, holding them tightly together.
Drizzle a bit of oil around the circumference to make sure the other side also gets crispy. Similarly, cook it for 3-4 minutes until golden.
When the galette has cooked, slide it off onto a plate and garnish with parsley to serve and the potato galette is done! 完成です!