Combine the buckwheat flour and sea salt in a large mixing bowl.
Add half the water (250 ml / 1 cup).
Mix with a spatula to form a smooth paste. Keep mixing with your spatula energetically for 15 minutes or until bubbles appear. Alternatively you can use a stand mixer with the paddle attachment and knead on medium speed for about 15 minutes.
Cover and reserve in the refrigerator for at least 24 hours and up to 72 hours.
After the resting time, thin the batter with the other half (250 ml / 1 cup) of water. Mix well until you get a smooth, watery batter.
Preheat a 9.5 inch (24 cm) crepe pan. When the pan is hot, lightly grease it with oil. You can use an oil soaked paper towel or cloth or a brush to do that. Make sure that the pan is greased well so that the galettes don’t stick.
Using a 2.7 oz (80 ml) ladle, scoop the batter. Add the measured batter into the pan. Swirl the pan as you add the batter. Work quickly, to spread the batter evenly around the pan all the way to the edges, so that you have a smooth edge. Continue to swirl the pan (and occasionally shake it gently), to move the batter around and fill the pan as evenly as possible. Once the bottom of the pan is evenly coated with the batter, place it back on the heat and cook.
Cook the galettes until the edges are starting to brown lightly, and they look a little crispy (about 3-4 minutes). Do NOT flip your buckwheat crepe. Buckwheat crepes only cook on one side, otherwise they dry out.
Add your fillings in the center of your crepe once it starts browning. Then fold your crepe, using a crepe spatula and serve immediately. And voilà. Bon appétit!