Wash and peel the russet potatoes.
Grate the potatoes using a box grater or food processor.
Soak the grated potatoes in cold water for about 10 minutes.
Drain the potatoes and pat them dry with a clean kitchen towel.
Finely chop the onion and set aside.
Heat vegetable oil in a large skillet over medium heat.
Add the grated potatoes and onion, spreading them evenly.
Season with salt and pepper.
Cook for about 5-7 minutes until golden brown, then flip.
Add butter if desired, and cook the other side until crispy.
Remove from the skillet and drain on paper towels before serving.