Cook the potato in a pan of gently boiling water just until it is easily pierced with a sharp knife, all the way through, but still intact and not mushy. Drain and cool the potato, remove the skin, and slice it into neat ¼-inch thick rounds.

Pour the olive oil into the skillet, set over medium heat, and scatter in the sliced onion and scallions. Cook for a minute then scatter the potato rounds in the pan. Toss and tumble the potatoes with the onion and scallions and season with the salt and grinds of black pepper. Cook for about 5 minutes, tossing frequently, until the potato rounds are lightly crisped and golden.

Pile the shredded Montasio on top of the vegetables. Slide a metal spatula under some of the potatoes and flip them over, incorporating some of the cheese. Turn all the slices over and over this way, until the cheese shreds are well distributed. With the spatula, clean the sides of the skillet and smooth the vegetables and cheese into a neat pancake-like disk, filling the pan bottom.

Lower the heat and let the frico cook, undisturbed, as the cheese melts and crisps, until the bottom is very brown and nicely crusted, about 5 minutes. Shake the pan to loosen the disk, put a large plate on top and invert, dropping the frico onto the plate, then slide it back in the skillet, top side down. Cook until the second side is crisp and brown, about 5 minutes more.

Slide (or invert) the frico onto the plate and blot up excess oil from the cheese with a paper towel. Slice into 6 wedges and serve immediately.