How to Make It
In a saucepan, cover the potatoes with 2 inches of water and bring to a boil. Simmer until tender, about 30 minutes. Drain and let cool to lukewarm, then peel. Grate the potatoes on the large holes of a box grater set in a large bowl. Add the cheese and toss well; season with salt and pepper.
In a 10-inch nonstick skillet, heat the oil. Add the onion and cook, stirring, until tender, about 8 minutes. Stir in the potato mixture. Gently press down to create a large potato cake. Cook over low heat until golden on the bottom, about 15 minutes.
Run a spatula around the side of and under the frico to release it from the skillet, then carefully pour off the excess oil. Gently flip the frico and continue cooking until golden and crisp on the second side, about 15 minutes longer. Slide the frico onto a large platter and blot with paper towels to remove excess oil. Season with pepper, cut into wedges and serve warm or at room temperature.