Combine soy sauce, vinegar, hot pepper flakes (if using), and sesame seeds in a bowl and mix it well with a spoon. Transfer it to a small bowl and set aside.

Combine green onion, leek (if using), zucchini, green chili pepper (if using), onion, and sweet potato in a bowl. Add flour, salt, and ¾ cup water. Mix it well with a wooden spoon.

Heat a large non-stick skillet over medium high heat. Add about 2 tablespoons vegetable oil and swirl it around to coat the skillet evenly.

Add the batter to the skillet and spread it out evenly. Turn down the heat to medium and put sliced mushroom on top. Gently press them in.

Cook for 4 to 5 minutes until the bottom is light golden brown. Turn or flip over the pancake.

Increase heat to medium high and add 1 to 2 tablespoons of vegetable oil along the edges. Cook for 3 to 4 minutes until both sides are light golden brown.

Flip it over one more time and cook another 2 minutes.

Transfer to a large plate and serve right away with the dipping sauce.