Prick the sweet potatoes all over with a fork and place in a 200°C oven. Bake for 40 minutes until soft.
While the potatoes are cooking heat the olive oil over a medium heat and add the onion. Cook for 2 minutes before adding the garlic. After another couple of minutes add the beef followed by the aubergine, mushrooms and yellow pepper. Turn the heat up a little and cook until the veg is soft and the mince is browned.
Add the tin of tomatoes and beef stock along with all the spices and cook for 15 minutes. Remove the sweet potatoes from the oven and, once they're cool enough to handle, peel them. Place the flesh in a large bowl and mash with a knob of butter.
Using a slotted spoon place the beef mixture into a baking dish and top with the sweet potato. Run a fork over the top to make grooves in the potato. Place in the oven for 10 minutes and serve.