Cut the goya in half, remove seeds and pith, and slice thinly. Sprinkle with salt and sugar, massage well, and let sit for 10 minutes. Drain excess water.

Slice the onion thinly and soak in water for 5 minutes. Drain well. Drain the tuna can.

Wrap the firm tofu in double-layered kitchen paper, place on a microwave-safe plate, and heat at 600W for 2 minutes. Let it cool.

In a bowl, mash the tofu with a whisk until smooth. Add the sauce ingredients (mayonnaise, ground sesame, miso) and mix well.