Preheat the oven to 425°F. Place the potatoes on a rimmed baking sheet and drizzle over the olive oil. Toss to coat then spread the potatoes out on the baking sheet and bake for 20 to 25 minutes, until golden brown and tender. Cool slightly.
While the potatoes are cooking, in a small bowl combine the mayonnaise, sour cream, harissa, and salt and pepper.
Serve the potatoes warm with the dipping sauce.
In patatas bravas the potatoes are usually cubed and boiled, and then deep fried, but for the purposes of our little world of crisp potatoes oven-roasting is just fine. The sauce usually consists of vinegar, red pepper, tomatoes, spices and olive oil, and sometimes an aioli, or mayonnaise is served alongside as well.