Preheat and grease a Takoyaki pan.
Combine Takoyaki flour with the egg and water to create a batter. Once you reach a homogenous consistency, transfer the batter into a squeeze bottle, piping bag, etc as per convenience.
Mix the filling ingredients together, the octopus with fish sauce and if you have it, the green onions cut into small pieces and the nori flakes as well.
Pour batter into the mold up to the brim. Add in the filling immediately (the octopus).
While the batter is slightly cooked, but moldable mold it into the shape of a ball until the ball is brown. With a designated takoyaki maker this is easily done by turning the balls in the round holes.
Transfer the balls once they're brown all around into a dish. You can top with your favorite toppings, serve it with your favorite condiments and you're done!