In a large bowl, stir together grated potatoes and onion, and season, to taste, with salt and pepper.
Melt 2 tablespoons of the butter in a 9-inch nonstick frying pan over medium-high heat. When bubbles subside, add the potato mixture and tamp down to spread into an even layer. Place a piece of foil over the top of the potatoes, then weight them down by placing a heavy plate on top that is slightly smaller in diameter than the pan. Cook, undisturbed, until the bottom is well-browned, about 15 minutes. Lift an edge of the potatoes with a spatula to check.
To flip, remove plate and foil. Invert the plate over the top of the potato cake and, holding the pan and plate together with oven mitts, flip the whole operation so the potato cake lands on the plate. Melt remaining butter in the frying pan, then slide the cake, uncooked side down, back into the pan. Replace the plate and foil on the top, as before, then cook until underside is brown and crispy, about 10 more minutes. Remove plate and foil and slide cake onto a serving plate. Cut into wedges and serve plain or with sour cream and chives or green onion.
Copyright 2001, Tori Ritchie. All Rights Reserved.