Adapted from Lidia's Italian Table. I made a few changes to this recipe, including reducing the amount of garlic olive oil in half and adding some beans for protein. I've served this soup two different ways, either with the Parmesan cheese crostini on top, or as the original recipe calls for, simply stirring the bread directly into the soup and sprinkling the Parmesan cheese on top. Either way, this simple soup is unexpectedly flavorful. The arugula loses all its bitterness after cooking.
In a 4-5 quart soup pot, combine the potatoes and water. Salt the water lightly and bring to a boil. Reduce heat to medium, and cook potatoes, covered, until they are tender but still hold their shape, about 18 minutes.
Meanwhile, cut off any thick stems from the arugula and wash well to remove any grit and salt. Drain arugula and cut into 2 inch lengths. Stir the arugula into the pot and let boil for another 10 minutes.
Meanwhile, in a small skillet, heat the olive oil over low heat. Add garlic and crushed red pepper, and cook until garlic is just golden.
To make crostini, brush a little garlic olive oil on top of the bread slice and sprinkle with some Parmesan cheese. Toast until cheese melts.
To serve, ladle soup into bowls and drizzle with some garlic olive oil. Break apart the Parmesan cheese crostini and serve with the soup.