Peel the sweet potatoes. Shred them with a cheese grater using the large holes then spread the grated potatoes on a flat plate.
Slice thinly the onions and add to the potatoes.
Season onions and potatoes with half of the melted butter, salt and pepper.
Place a thin layer of shredded potatoes and the chopped onions in a 20 cm diameter non-stick frying pan brushed with butter, sprinkle with grated Emmenthal and top with the remaining vegetables. Cook on low flame for long time, until a thick crust is formed, flip the tortilla-looking pie, and insert some knobs of butter below it. Continue cooking with the lid on until golden also on the other side.
Sprinkle some drops of Aged Balsamic Vinegar of Modena PGI and the chopped herbs.
