Wrap potatoes in damp kitchen paper and microwave on a heatproof plate at 600W for 7–9 minutes. Cut cabbage into bite-sized pieces, add to a bowl with 1/3 tsp salt (extra), let sit for 5 minutes until softened, then rinse and squeeze out excess water.

Mash the hot potatoes roughly with a fork in a bowl and let cool slightly.

Add tuna and cabbage to the bowl, mix well. Add mayonnaise, salt, and black pepper, then mix thoroughly.