Boil whole potatoes for about 15 minutes until just tender, then let them cool completely. Cooling is important so they don’t turn mushy when grated. You can even refrigerate them ahead of time for easier prep.
Coarsely grate the cooled potatoes using a box grater. Some of the thin skin may come off naturally—just discard it. The texture should be chunky, not mashed, for the perfect crispy finish.
Heat a skillet with oil and butter over medium-high heat. Spread half of the grated potatoes evenly and press them down gently. Season with salt, pepper, and sprinkle green onions, then cook until the bottom turns golden.
Scatter all the grated Jarlsberg cheese over the cooked layer. Cover it with the remaining potatoes and press lightly to seal. This creates that signature cheesy center once melted.
Carefully flip the rösti onto a plate or board, then slide it back into the pan. Add more butter or oil if needed and cook until the other side is crispy and golden, about 5–7 minutes.
Transfer the rösti to a cutting board and let it rest briefly. Slice into wedges and garnish with extra chives if you like. Serve warm while the cheese is still perfectly melted.