1. Cook the potatoes in salted water.
2. Peel them and mash them into a dry puree.
3. Flake the tuna with a fork and mix it with the puree.
4. Add the butter, parsley, and gruyere cheese, season with salt and pepper.
5. Chill the mixture for at least 1 hour (or preferably prepare the mixture the day before).
6. Shape the mixture into flat patties.
7. Beat the egg in a bowl, dip the croquettes in it, then coat them in breadcrumbs.
8. Fry them for a few minutes in a deep fryer (with hot frying oil).
9. Drain them on paper towels and serve with a lemon wedge.