Wash and peel potatoes. In a large food processor, fitted with the shredding disc, shred the potatoes. Transfer to a large bowl and season with salt and pepper, tossing mixture with a fork.
In a 9 to 9 1/2-inch nonstick skillet heat 1 tablespoon butter and 1/2 tablespoon oil over moderate heat until foam subsides. Add potatoes, spreading them evenly and tamping them down with a rubber spatula to form an even cake. Reduce heat to moderately low and cook rosti until underside is golden brown, 10 to 12 minutes.
Slide rosti onto a large plate. Invert another large plate over rosti and invert rosti onto it. (Browned side of rosti should be on top.) In skillet heat remaining tablespoon butter and 1/2 tablespoon oil over moderately low heat until foam subsides. Slide rosti back into skillet, browned side up, and cook until underside is golden brown, 10 to 12 minutes. Slide rosti onto a serving plate and cut into wedges.
Recipe courtesy of Gourmet Magazine