Drain the tuna can. Cut the onion, carrot, broccoli, and potato into bite-sized pieces. Trim the shimeji mushrooms and separate them into small clusters.

Mix the tuna, mayonnaise, miso, and pepper in a bowl.

Boil water in a pot. Add broccoli and mushrooms, cook for 1-2 minutes, then remove. Add carrot and potato, cover, and simmer on low heat for 5 minutes. Add onion and cook until softened. Drain well.

Combine the cooked vegetables with the sauce in a bowl.