Peel and finely chop the onion. Peel and dice the carrot and sweet potato into small 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 prepped veg and garlic. Season with salt and pepper. Cook for 5 mins, stirring occasionally, till the onion is soft. Add 1 tsp each fennel and cumin seeds and cook for 1 min, then add 1 tsp 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 ½ the lemon. Taste and season, adding a little more lemon juice, if you wish.
Ladle the soup into 2 warm bowls. Serve with the coriander leaves scattered over the top.