Preheat the oven to 190°C.
Steam the sweet potatoes for 20 minutes until tender, and set aside.
Put the oil in a large non-stick pan on medium heat.
Add the onion, garlic, cumin and chilli flakes and cook until starting to brown. Add the beef and stir continuously until this also starts to brown, about 5 minutes.
Reduce the heat to low. Stir in the peppers, mushrooms, tomato purée, peas, sugar snap peas, cinnamon, nutmeg and Worcester sauce. Cover and simmer for 10 minutes. Meanwhile, prepare your topping. Mash the cooked sweet potatoes in a bowl with the butter, salt and pepper.
Pour the beef and vegetable mixture into a medium-sized baking dish.
Spread the mashed sweet potato over the beef and sprinkle the Cheddar cheese on top.
Put in the oven and bake for 15 to 20 minutes until the top is golden brown and the cheese is bubbling.