Set a bowl over a scale and add the buckwheat flour, salt, egg, and oil. Pour in half (250 g) of the water and mix together for 3-5 minutes, ensuring there are no lumps. Gradually add the remaining water while stirring until combined. Let rest overnight in the fridge or up to 3 days.
When ready to cook, set the largest nonstick skillet or carbon steel pan over medium heat. Rub some oil onto the pan. Once hot 190-220C, pour or ladle roughly 70 grams of batter into the bottom fourth of the pan. If you have a rondelle (crepe stick) quickly thin it out as best you can. This is the tricky part! Depending on the size of the pan and how much batter is used will determine the thickness of the crepe.
Let cook on one side until lightly browned and it lifts freely off the pan. Set aside on a plate and cook the remaining galettes. Store the parcooked galettes in the fridge for up to 4 days.
To crisp, turn the heat to medium. Rub a little more oil on the pan and add the galette browned side up. Add knobs of butter to the galette. Once they start to melt, flip it over. Let crisp until deeply brown. Roll up with toppings or make a galette complet (below)
Galette Complet (Ham, Cheese + Egg)
Set a pan over medium heat. Rub a little oil on the pan and add the par-cooked galette browned side up. Add some knobs of butter. Once they start to melt, flip the galette over to let crisp. Crack the egg in the center and break up the white so it cooks evenly. Once the white is almost set, add the shredded cheese, deli ham, and mustard, if using. Let cook until the bottom is deeply browned. Fold the 4 sides of the galette up so the egg is still showing (see the video for an example).
Serve with chives and black pepper over the top. Devour.