Step 1Pour the chicken broth into a large saucepan and bring it to a simmer. Add the potatoes and cook, covered, until they are very tender, 20 to 25 minutes.
Step 2Wash and drain the arugula. Add it to the saucepan. Stir to combine and simmer until the arugula is completely wilted, 1 to 2 minutes.
Step 3Puree with an immersion blender right in the saucepan, or in a food processor in one or two batches, transferring each batch to a clean saucepan or warm tureen. Stir in the pepper and swirl in the creme fraiche or olive oil.