Bake the sweet potatoes: Preheat oven to 200°C (400°F). Pierce the sweet potatoes with a fork and bake for about 45 minutes, or until completely soft.
Start the masala: Heat oil in a pan. Add mustard seeds and cook until they pop, then add cumin seeds and cook until fragrant.
Cook the base: Add onion, curry leaves, and dried red chilli. Cook until onions are translucent. Stir in garlic and ginger and cook briefly until the raw smell disappears.
Add spices & tomatoes: Add chilli powder, coriander, cumin, turmeric, and salt. Mix and cook for a few seconds. Pour in chopped tomatoes and tomato purée, mix, cover, and cook for 5 minutes.
Add chickpeas: Stir in the chickpeas, cover again, and cook for another 5 minutes until slightly thickened.
Prepare the skins: Slice baked sweet potatoes in half and scoop out most of the flesh, leaving a thin layer inside. Place skins skin-side up, drizzle with olive oil, and season with salt and black pepper. Bake for 7-10 minutes.
Finish the filling: Add the scooped sweet potato to the masala along with chopped coriander and lemon juice. Mix, lightly mashing to create a thick filling.
Fill and bake: Spoon the mixture into the skins and sprinkle with vegan cheese if using. Bake for 10–15 minutes until hot and lightly golden.
Serve: Serve warm topped with extra coriander and diced red onions.