Wash and dry the potatoes. Cut into 1/4" chunks and toss with the olive oil and salt. Spread into a single layer on a baking tray, covered with parchment paper if desired. Roast until potatoes are browning and crisp, 35 to 45 minutes.
While potatoes are still warm, toss with harissa, feta, and sprinkle with a bit of cilantro before serving.
Notes
+ My harissa recipe is a cheat method and not near as spicy as a traditional harissa recipe. If you want to try your hand at a traditional recipe, here is a good one from Saveur.