Peel and finely chop the onion. Peel and cut the carrot and sweet potato into cubes. Peel and thinly slice the garlic.
Set a large pan over a medium heat. When warm, drizzle in 1 tbsp olive oil and tip in all the veg. Season. Cook for 5 mins, stirring occasionally till the onion is soft. Scatter in 1 tsp each of fennel and cumin seeds and cook for 1 min. Add 1 tsp of the turmeric.
Crumble the stock cube into the pan. Pour in 800ml boiling water. Zest the lemon and add to the pan. Tip in the bulgar wheat. Bring to a gentle bubble. Lower the heat and pop on a lid. Cook for 10 mins till the bulgar is tender.
Pick the leaves from the coriander. Finely chop the stalks. After the soup has cooked for 10 mins, add the coriander stalks. Squeeze in the juice from half the lemon. Taste and season. Add a little more lemon juice if you wish.
Serve the soup in warm bowls with the coriander leaves scattered over the top.