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. 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 up 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 the sliced mushroom on top. Gently press them in with a wooden spoon or spatula.

Cook for 4 to 5 minutes until the bottom turns crunchy light golden brown. Grab the handle of the skillet and twirl it around so the pancake moves and is cooked evenly underneath.

Turn or flip over the pancake. Increase the heat to medium high and add 1 to 2 tablespoons of vegetable oil along the edges of the pancake.

Lift up one edge with your spatula and tilt the skillet so the oil flows underneath the pancake. Cook for 3 to 4 minutes until both sides turn light golden brown, occasionally pressing down with the spatula.

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

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