Wash the potatoes and wrap them wet in aluminum foil.
Preheat the oven to 200 °, fan oven to 160 °. Cook the potatoes in the oven for 1 hour, then peel them and press them through a potato press. Season the potato snow well with salt, then knead with flour, semolina and the egg to form a smooth dough. Shape the dough into rolls approx. 2 cm thick and divide into 1 cm small pieces, roll into balls and flatten with a gift, or pull over a gnocchi board, so that the typical gnocchi shape is created. Place the finished gnocchi on a well-floured baking sheet or lined with baking paper.
For the sauce, rinse the salt capers well in a sieve, then place them in warm water.
Drain the tuna in a colander and cut into bite-sized pieces.
Heat the olive oil and butter in a saucepan or pan. Sweat the garlic in it. Reduce the heat and add the tuna, lemon juice, drained capers and olives. Deglaze with nolly prat and stock.
Put the gnocchi in plenty of boiling salted water and cook until they rise to the surface. Lift out with a slotted spoon and place in a pasta plate, spread the sauce on top, and serve sprinkled with parsley and parmesan.