Preheat the oven to 200°C.
Put the sweet potatoes onto a baking sheet, then put the tray in the oven.
Bake the potatoes until their flesh is tender (test it with a skewer).
Take them out of the oven and let them cool. You can refrigerate them for up to 3 days or continue with the recipe now.
Cut the sweet potatoes in half, lengthwise, then scrape out the flesh into a bowl.
Brush their inside with oil and put them back ion the tray then in the oven for 5-10 minutes, until the skin becomes crispy.
Meanwhile, finely chop the onion, garlic, carrot and red pepper.
Heat the oil in a saucepan and sauté the chopped veggies until soft.
Add the spices and the tomato purée. Season with salt and pepper. Stir for a minute then add the passata and lentils and enough water to cover the vegetables.
Cook for about 15 minutes, until you a get a nice, silky sauce.
Remove the star anise, bay leaf and cinnamon piece. Add the chopped fresh coriander. Add the sweet potato flesh, and with a fork mash the stew until you get a thick sauce.
Add the cooked rice and mix well. You should get a thick and moist (not wet!) mixture.
Fill the baked sweet potato skins with the mixture. You can serve them immediately or you could scatter some grated cheese on top and bake for another 5 minutes until the cheese has melted.