Heat 1-inch salted water (1/2 teaspoon salt to 1 cup water) to boiling. Add potatoes. Heat to boiling. Reduce heat. Cover and cook until tender, 30 to 35 minutes. Peel and shred potatoes or cut into 1/4-inch strips. Heat margarine in 10-inch skillet until melted. Add potatoes, onion and cheese. Sprinkle with salt and pepper. Cook, uncovered, over medium heat, turning frequently, until potatoes start to brown, about 10 minutes. (Add 1 to 2 tablespoons margarine to prevent sticking if necessary.) Press potatoes with spatula to form flat cake. Sprinkle with water. Cover and cook over low heat, without stirring, until bottom is golden brown and crusty, about 10 minutes. Place inverted platter over skillet and invert potatoes onto platter.