Wash and scrub the sweet potatoes. Place in a large pot with cold salted water and bring to a boil over medium heat. Reduce heat to low and simmer until fork tender but al dente, about 15 minutes.
Remove, cool slightly, peel (if boiling), and cut into 1 ½ - inch / 4 cm chunks. Transfer to a serving bowl. (Alternatively, you can peel and cube the sweet potatoes and either roast or grill them under the broiler, on a sheet pan tossed with a little olive oil and salt).
Peel, halve and thinly slice the onion. Of using scallions, trim and cut into thin rounds.
Coarsely chop the arugula.
Place the arugula, and onions or scallions in the bowl with the sweet potatoes. Season to taste with salt and toss with the olive oil and vinegar. If desired. Add the crumbled feta or goat’s milk cheese and serve.