Grate potatoes and onion using shredding disk of food processor or hand-held grater. Squeeze out excess liquid.
Combine potatoes, onion, and remaining ingredients except oil in medium mixing bowl.
Heat oil in 10-inch nonstick skillet over medium-high heat. Spread potato mixture evenly in skillet. Reduce heat to medium; cook covered until top is nearly set and bottom lightly browned, 6-8 minutes. Invert cake onto platter and slide back into skillet. Cook until bottom is golden brown, about 4 minutes. Cut into 8 wedges. Transfer to serving plate and serve hot.