Rösti is originally a dish of Swiss farmers from the Bern and Zurich cantons that has spread throughout the country. These fritters are made from coarsely grated potatoes, and their magic lies in the crispy crust and tender interior.
The most traditional method uses partially boiled potatoes from the day before (Gschwellti). This ensures the Rösti won't fall apart in the pan and will be perfectly crispy.
Rösti is most often made as a large fritter covering the entire pan, which is then sliced into portions.
Boil the potatoes in their skins in salted water until half-tender (about 15-20 minutes). They must not be overcooked!
Drain the water and let the potatoes cool completely, ideally overnight in the refrigerator. (This step is crucial for crispiness, as the potatoes will firm up and lose excess moisture).
Peel the cooled potatoes and grate them coarsely (use a coarse grater, suitable for apples).
If using onion, briefly sauté it in butter and let it cool.
Mix the grated potatoes with the sautéed onion (if using), salt, and pepper. If adding cheese or bacon, mix them in now. Do not add eggs or flour - Rösti does not require them.
In a large frying pan (approx. 20-25 cm diameter), heat the butter and oil over medium heat.
Place the potato mixture into the pan and press it down evenly with a spatula to form a uniform, flat fritter (about 1.5–2 cm thick).
Fry the Rösti slowly for about 8-10 minutes until a thick, golden-brown, and crispy crust forms on the bottom side.
Place a plate larger than the pan over the Rösti and quickly flip the pan (so the fritter remains on the plate with the unfried side up).
Add the remaining butter to the pan and let it melt.
Slide the Rösti back into the pan, unfried side down, and fry for another 8-10 minutes until crispy and golden on the second side.
Serve the hot Rösti immediately, sliced into portions, as a side dish for meat (e.g., Zürcher Geschnetzeltes) or as a main course (e.g., with a fried egg).