Cut the new potatoes into quarters and soak them in water. Drain and place them in a microwave-safe container. Sprinkle with salt and 1 tbsp of water (not listed in ingredients). Cover loosely with plastic wrap and microwave at 600W for 6 minutes until tender. Drain and let cool slightly.

Finely chop the onion and soak it in water for about 5 minutes. Drain and squeeze out excess water using kitchen paper.

In a bowl, mix the mayonnaise, soy sauce, and pepper. Add the potatoes, onion, and canned tuna, then mix well.