Cut a cross on top of each sweet potato, just through the skin, with a sharp knife. Place the sweet potatoes on a large plate and place in the microwave oven. Microwave on full power for about 15 minutes, until tender when a knife is inserted. We used an 800 watt microwave oven, but you may need to adjust this timing depending on the power of your microwave. Alternatively, you can bake the sweet potatoes in a moderate oven for around 1 hour or until tender.
Remove from the oven and leave to stand for 5 minutes while you make the chimichurri sauce. Place the oregano leaves, coriander leaves, flat leaf parsley and chilli flakes (if using), in a mini chopper, or use a stick blender to finely chop. Alternatively, you can use a large sharp knife to chop finely. Take care not to over process and turn the herbs into a puree.
Transfer the finely chopped herbs and chilli flakes into a small bowl. Stir in the lemon zest, lemon juice, red wine vinegar, garlic granules (1-2 tsp, but adjust to suit your taste), water and granulated sweetener. Taste, and season with salt and black pepper if needed.
Open up the cooked sweet potatoes and fill with the chopped cherry tomatoes. Place a couple of spoonfuls of yoghurt on top of the tomatoes in each sweet potato.
Add a spoonful or two of chimichurri sauce on top of each and serve at once with a small mixed salad or other accompaniment of your choice.