1. Squeeze the grated potatoes to remove excess moisture. In a large bowl mix the potatoes, onion, cabbage, flour and egg. Season with salt and pepper.
2. Heat the oil and butter in a large pan. Spread the potato mixture in the pan and compact it with a wooden spoon. Fry over low heat for about 10 minutes or until the rösti turns golden brown and becomes denser.
3. Use a large egg lifter to carefully turn the rösti over. Fry until golden brown on the other side (another 5-10 minutes). Carefully transfer to a serving plate.