Grate potatoes and onion by hand or with grating disk of a food processor; drain in colander or strainer. Combine potatoes and onions in a large bowl with eggs, salt, pepper and bread crumbs or matzo meal. Put about 1/8 inch oil in a large, deep skillet, either nonstick or seasoned cast iron; turn heat to medium-high. When oil is hot (it will shimmer) put all the batter in pan, and smooth the top. Cook, shaking pan occasionally; adjust heat so mixture sizzles but does not burn. Continue until bottom is nicely browned, at least 15 minutes. To turn, slide cake out onto a large plate, cover with another large plate and invert. Add a little more oil to pan if necessary, and slide pancake back in, cooked side up. Cook 15 minutes or so longer, until nicely browned. Serve hot or warm.