Heat your oven to 220°C/Fan 200°C/Gas 8. Put a roasting dish on the top shelf to heat up.
Peel your sweet potatoes. Cut into 3cm chunks. Toss with the oil and some salt. Tumble into your warmed roasting dish. Roast for 20 mins or till tender and nicely coloured.
Sprinkle the Cajun seasoning over the roasted sweet potatoes.
Halve your peppers. Scoop out the seeds and stem. Finely dice, then toss with the watercress.
Finely chop your chives. Swirl through the yogurt. Season to taste. Dollop the yogurt dressing over the salad. Scatter the spiced sweet potatoes over the top. Serve any remaining dressing on the side.