Preheat your oven to 220C / 200C / gas mark 7 / 425F.
Mix together the cumin, paprika, chilli, salt, garlic, olive oil and lime juice in a large bowl.
Add the cubes of sweet potato and stir to coat thoroughly. (Or use your hands!)
Place the sweet potato cubes on a large non-stick baking tray, ensuring the cubes are well spread out (use 2 baking sheets if necessary) and in a single layer.
Roast in the oven for 20 minutes.