Make the batter: mix together sourdough discard, water, salt, and cornstarch. Set aside.
Make the egg layer: scramble egg with a splash of water. Mix in scallion. Set aside.
Make the dipping sauce: mix together soy sauce, black vinegar and ginger. Set aside. Add sesame seeds & scallions right before serving.
Heat up a nonstick pan on medium high heat. Generously brush the pan with oil to prevent sticking. Pour half of the batter into the pan and spread into a thin layer using a flat spatula or if you are using a skillet lift it at an angle and rotate it around so you create an even thin layer. When the top starts setting (it will start looking translucent), mix your egg layer again and pour half of it on top of the crepe layer. Evenly spread the egg mixture using a spatula or by lifting and turning your skillet.
Optional: When the egg layer sets, carefully flip the crepe so the egg layer is on the bottom. This will be very quick ~10 seconds. Flip the crepe so the egg side is back on top.
Turn off the heat and using a spatula or chopsticks, roll up the crepe. Alternatively, you can transfer the crepe onto a cutting board and roll it up there. Repeat with the other half.
Cut and serve warm with dipping sauce.