Snap or cut beans into 2-inch lengths, removing ends.  Rinse and drain.
Peel potatoes. (Potatoes soak up seasoning better when peeled.)

Heat oil in large, heavy pot.
Add beans, salt, pepper, and sugar and turn heat to low, cover, and let simmer with ham and onions for about 20 minutes.
During this period, beans should release some liquid and cook down a bit.
If not, add a little water and continue simmering, covered, for about 40 more minutes.
When beans cook in their own juices, their flavor is more intense than when they are boiled in water.

After beans have cooked for an hour, add potatoes and enough water just to cover the vegetables.
Continue simmering, covered, until the potatoes are done, about 35 to 40 minutes.