Toss the diced sweet potato chunks in 1 tbsp oil, and lay them out in a single layer on a baking tray. Roast at 190°C (Gas Mark 5 / 375°F) for around 30 minutes, or until soft and slightly crispy around the edges.
Meanwhile, add the remaining 1 tbsp oil in a large saucepan, and add the diced onion. Cook over a medium heat for a few minutes, until soft. Add the tinned tomatoes, kidney beans and spices, and bring to a simmer. Cook for at least 10 minutes, until the tomatoes have thickened and the mixture looks nice and rich.
Add the roasted sweet potato pieces to the pan, and mix well. Cook for a few more minutes, then season generously, and serve with your choice of toppings.