In the vegetable mill, mash potatoes and egg yoks.
In the electric mixer, mix tuna with butter, lemon juice and capers (if you like them) to obtain a thick mousse. (*) You can substitute butter with a little bit of olive oil to make the recipe pareve.
Add salt if necessary, then roll the mousse inside a clean kitchen towel.
Place the mousse in the fridge and let it rest there for a couple of hours.
Serve the fish pâté cold and decorate it with capers and lemon slices.