Place the sweet potatoes in a roasting tin with half the oil and season. Toss well, then roast in a preheated oven, 220˚C (425˚F), Gas Mark 7, for 20–25 minutes, until golden and tender.
Meanwhile, combine all the meatball ingredients in a bowl, adding some salt and pepper. Roll into 20–24 balls. Heat the remaining oil in a large pan and cook the meatballs over a medium-high heat for 3–4 minutes, shaking the pan occasionally, until browned. Transfer to a plate and set aside.
Return the oily pan to the heat and cook the onion, pepper and garlic for 7–8 minutes, until softened and lightly coloured. Add the spice mix and stir over a medium heat for 1 minute. Pour in the passata, add a pinch of salt, pepper and sugar and simmer for 5–6 minutes to thicken slightly.
Add the meatballs to the sauce and simmer for a further 6–7 minutes, until cooked and the sauce has thickened. Serve with the sweet potato wedges and a dollop of soured cream, if desired.