Whisk in the milk, eggs, flaked trout and puréed Yukon Gold potatoes until the batter is emulsified. Season with salt and pepper.
Heat the takoyaki pan to medium heat and brush each space well with oil.
Pour batter into each hole of the pan, filling it completely.
Cook for just over 1 minute or until set, then carefully use a chopstick to flip each ball 90 degrees. Pour more batter around each ball until just filled.
Rotate the balls, while cooking, adding batter to form a smooth ball.
Cook the balls until the batter is cooked through and golden brown, for a total of 5-8 minutes.
Garnish with Japanese mayonnaise, sriracha, furikake, scallions and red bell pepper threads.