Rinse the very thinly sliced potatoes until the water runs clear to wash off the starch, then drain well (this keeps them from sticking together into a gummy mass).
Confit the potatoes gently in oil heated to 160C/320F until tender.
Caramelize the thinly sliced onions with a pinch of salt (the salt helps release their liquid) until deeply golden. Aim for roughly two parts potato to one part onion.
Beat the room-temperature eggs with plenty of black pepper, then fold in the confit potatoes and caramelized onion.
Pour into a hot oiled pan; you should hear a light sizzle. Cook to a deep caramelization on the outside while keeping the middle soft and gooey.
Slide the tortilla onto the back of a tray, then slide it back into the pan to brown the other side. Roll and shape the edges, then flip out.
Serve with a small quenelle of mayonnaise and a scattering of chopped chives.
