Boil potatoes for 15 minutes until just tender. Let cool completely.
Coarsely grate the potatoes, removing any loose peel.
Heat a skillet with oil and butter. Add half the grated potatoes and press down.
Season with salt, pepper, and sprinkle with green onion. Cook until golden.
Add all the cheese, then the remaining potatoes. Press gently.
Invert onto a plate, add more oil/butter to skillet, and return rösti crispy side up.
See full steps with tips & photos → https://theboatshedcafe.com/cheesy-potato-rosti-with-jarlsberg/