Mix bread flour, tapioca flour, salt and water together and stir until it's very smooth. Add in scallion, sesame seeds and stir to combine.
Heat a nonstick pan with medium heat, then spray or brush on a thin layer of cooking oil.
When the pan start to warm up a little bit, pour in the batter and spread it with back of a spoon or a pastry brush to an even 7-8 inch round. It's very important to do this before the pan gets hot, otherwise it will be hard to spread.
Cook for 2 minutes and flip to cook the other side until some brown spots develop on that side (about 1.5 to 2 minutes), then remove the pancake to a plate.