Cut the sweet potatoes in half, then cut each half into 4 wedges and place in a large mixing bowl. Cut the baking potatoes in half, then cut each half into 6 thick wedges and place in the bowl. Drizzle over the olive oil, then toss well to coat all the potato wedges.
Transfer the potatoes to a large baking sheet or oven tray in a single layer. Sprinkle over the Cajun spice. Roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 30–35 minutes until golden and cooked through. Turn on to a serving platter and sprinkle with the parsley.
Mix the yogurt, soured cream, chives and Parmesan in a small mixing bowl. Serve the dip with the warm potato wedges.