Peel and fine-grate potatoes, or dice and use a blender. The potatoes need to be a real purée. It’s ok to add water to blend, as later you MUST put the potatoes into a fine mesh strainer and let the water seep out.
After 2 or 3 strainings to remove excess water (takes 30 minutes or so), put potato “mud” into a bowl and add egg, salt, pepper, and flour and mix well.
Add sautéed onion-cabbage mixture to potatoes, mixing well.
Heat thin layer of oil in electric frypan at 300° F, drop large spoonfuls of mixture into pan and smooth out to a thin pancake about 3-4” in diameter.
Cook about 5-7 minutes a side until nicely browned and crisp.
Place on a paper towel lined plate, keeping warm as you do other batches.