Preheat the oven to 218°C. Cut the potatoes into long wedges about 1-inch thick. Toss the potatoes with the olive oil and Cajun seasoning. Heat an ovenproof large non-stick frying pan over medium-high heat and add the potatoes in a single layer. Cook, turning, until browned on all sides, about 15 minutes. Slowly add 60ml water to the pan and transfer the entire pan to the oven.
Continue to cook the potatoes, turning once, until the potatoes are tender and the outsides are crisped, about 20 more minutes. Meanwhile, mix the yoghurt with the mustard and a pinch salt. Serve the potatoes warm with the mustard dipping sauce.