Pre-heat the oven to 180ºC.
Spray a non-stick baking tray with low calorie cooking spray.
Cut the sweet potatoes into 2 cm evenly sized cubes and place on the baking tray
Spray the sweet potato cubes with low calorie cooking spray and place in the oven for 25 minutes. After 25 minutes, turn the sweet potato cubes over and spray with low calorie cooking spray again. Return to the oven for a further 25 minutes until tender when pierced with a knife and just beginning to brown at the edges.
Remove from the oven and leave to cool.
Place the yoghurt and harissa paste in a medium mixing bowl and mix well.
When the sweet potatoes have cooled, add them to the bowl with the spring onion and season well with salt and pepper. Stir until the sweet potatoes are all coated and serve.