1. Scrub potatoes and cook in boiling water until fork tender, but not mushy.
2. Cool potatoes and slice into thin round slices.
3. Heat the margarine and oil in a large heavy skillet and add the onions, cooking for about 3 minutes, or until just starting to get tender.
4. Add the potatoes, season with the salt and pepper and cook 5 minutes, turn, then cook 5 more minutes or until browned.