Peel potatoes and cut into 1-inch pieces. In a 4 quart saucepan boil potatoes, garlic, and water, skimming froth, until potatoes are very tender, about 15 minutes.
Set broiler rack about 1-inch from heat and preheat broiler.
While potatoes are boiling, in a bowl, sprinkle bread with oil and salt to taste and toss. On a baking sheet arrange bread in one layer as close together as possible and sprinkle with half of Parmesan. Broil bread until Parmesan is melted, about 1 minute.
Transfer potatoes with a slotted spoon to a bowl, reserving cooking liquid in pan, and with a potato masher mash coarse. Stir potatoes and broccoli rabe into reserved cooking liquid and simmer, partially covered, stirring occasionally, until broccoli is tender, about five minutes. Stir in remaining Parmesan and salt and pepper to taste.
Serve soup with croutons.
(recipe courtesy of Gourmet Magazine)