Cook the potatoes by either baking them or covering them with water in a saucepan and boiling for 15 minutes, or until just tender. Cool completely, or refrigerate for up to a few days.
When you're ready to make your rösti, coarsely grate your potatoes on the large holed side of a box grater. Much of the thin potato peel will sort of come off in your hand - just pitch it. Set a 9 or 10-inch skillet over medium-high heat, add a generous pour of oil and a chunk of butter (a tablespoon or two) and spread half the grated potatoes into the bottom of the pan, patting them down so they're even. Season with salt and pepper and sprinkle with green onion. Continue to cook for a few minutes, until the rösti starts to turn golden on the bottom.
Scatter all the Jarlsberg evenly overtop, then top with the remaining grated potatoes, evening them out as well. By now the potatoes should be deeply golden on the bottom. Carefully invert the rösti onto a large plate or cutting board, add more butter or oil to the pan and slide the rösti back in, crispy side up (or loosen and slide it out onto a plate, then invert back into the pan). Cook for another 5-7 minutes, until well crisped on the bottom and heated through so that the cheese has melted.
Slide the rösti out onto a cutting board and if you like, top with more green onions or chives. Serve warm, in wedges. Serves 8.