Peel and cut potatoes into cubes. Place in a pot of cold water. Bring to boil. Boil for 12 minutes.
While potatoes boil, prepare the rabe. Wash it well. Discard stems. Dice leaves and tender stems into small pieces.
When potatoes are done, add rabe to the boiling water and boil with potatoes for 3 minutes. Drain and set aside.
Clean and dry the pot used to cook the potatoes. Dice the onion, ham, garlic and sage. Pour olive oil into pot. Heat. When oil is hot, sautee onions until brown.
Add ham and continue sautee 1-2 minutes more. Add garlic and sage. Sautee for another minute.
Return potatoes and rabe to the pot with the ham, sage, onion and garlic. Mix together,mashing the potatoes a little with the spoon.
Serve warm and add grated parmesan or romano cheese if desired.