Pre-heat your oven to 200°C. Line a baking tray with baking parchment and set aside. Place a large sweet potato in the microwave (prick it first) for 8-10 minutes.
Chop the root end off the onions (do not cut the other end off!) and peel. The top of the onion will hold it together as you slice it on the mandoline.
Select the finest blade for your mandoline and slice the onions. Watch your fingers and remember to always use the guard! If you prefer, you could also grate the onion instead.
Use your hands to squeeze the juice from the onions over the sink and then place them into a large bowl.
Combine the ground coriander, ground cumin, garam masala, chilli powder, spring onion, and salt and pepper in a bowl, and add to the onion mix. Stir well.
Next grate in the cooked sweet potato - the skin will just fall away.
Get your hands in and mix until the mixture forms a ball. Split the mixture into 12 and make into small balls using your hands.
Place on the baking parchment and spray well with low calorie cooking spray
Place in the oven and cook for 20 minutes. After 10 minutes remove from the oven, spray them again with the low calorie cooking spray and return to the oven for the remaining 10 minutes.