How to Make It
In a medium saucepan of boiling salted water, cook the potato and carrot over high heat until tender, about 5 minutes. Drain and let cool.
Carefully scoop the hard-cooked egg yolks into a large bowl and mash with a fork. Add the mayonnaise, the 1 tablespoon of parsley and the vinegar and season with salt. Fold in the potato, carrot, celery, onion and tuna. Mound the mixture in the egg halves and arrange on a platter. Garnish with parsley and serve.