Start by boiling or baking the potatoes until just tender but not soft.I like to keep the skins on because they add texture and flavor.Let them cool completely before grating to avoid mushiness.
Using a box grater, coarsely grate the cooled potatoes into a bowl.Some of the peel will naturally fall off—just discard it.Make sure the shreds stay fluffy and not mashed together.
Heat oil and butter in a skillet over medium-high heat.Spread half the grated potatoes evenly and press them gently.Season with salt, pepper, and sprinkle green onions on top.
Scatter all the grated Jarlsberg evenly over the base.Cover it with the remaining grated potatoes and press lightly.This creates a sealed layer that keeps the cheese inside.
Once the bottom is golden, carefully flip using a plate.Slide it back into the pan with more butter or oil.Cook until the second side is crispy and the cheese melts fully.
Transfer the rösti to a cutting board and let it rest slightly.Slice into wedges for easy serving and better presentation.Top with extra chives if you like a fresh finish.