Bring a large pot of water to a boil, and add the cauliflower and potato. Cook until tender, 10 to 12 minutes. Drain, and set aside.
Bring another saucepan of water to a boil. Trim the arugula, discarding the tough stems (you should have 2 cups leaves), and rinse it well. Cook until tender, 2 minutes. Then drain it thoroughly and chop coarsely.