1. Grate the potatoes in a food processor (or using a grater).
2. Place potatoes in a bowl, drain any liquid.
3. Heat 1 tablespoon oil in a medium skillet over medium high heat. Saute the onion until just barely translucent, then add the mushrooms and cook until they are done.
4. Add the mushrooms and egg to the potatoes. Mix them together. Then add the flour and mix again.
5. In a large skillet, heat 2 tablespoons oil over medium high heat.
6. Drop the mixture on the skillet 1/4 cup at a time. Slightly flatten with a spatula to 1/2 inch (1cm) thick (in this recipe you can also scoop the mixture with your hands and form into pancake shapes). You may need to cook in batches.
7. Fry approximately 5 minutes on each side, until golden brown. Serve hot, topped with sour cream.