Heat the oven to 200°C (400°F/Gas 6). Put the whole sweet potatoes on a baking tray and bake for about 40 minutes, or until very soft. While still hot, cut in half lengthways and carefully scoop most of the flesh into a bowl, leaving a layer of flesh on the skin. Be careful not to tear the skin. Add the parmesan cheese, chilli flakes, spring onion, 2 tablespoons of the sour cream, half the lemon zest and all the herbs to the sweet potato flesh. Mix well and season with salt and pepper.
Spoon the filling back into the sweet potato cases. Don’t worry that the skins seem very soft as they will crisp up when returned to the oven. Sprinkle over the breadcrumbs and drizzle with olive oil. Bake for 10 minutes, or until the topping is golden and the filling warmed through. Meanwhile, mix the remaining sour cream with the remaining lemon zest.
Leave the sweet potato halves whole, or cut in half if you are serving these as finger food. Serve with the sour cream on the side.