Peel and chop the onion. Peel and grate or crush the garlic cloves. Trim and peel the carrots. Finely chop it. Finely slice the celery sticks. Peel and cut the sweet potato in to 2 cm cubes.
Thickly slice the rainbow chard leaves and stems. Zest and juice the lemon.
Place a large pan on the heat. When warm, drizzle in 2 tbsp olive oil. Add the onion, garlic, carrots and celery. Fry gently for 3 mins. Sprinkle in 1 tsp of the cumin seeds and cook for 1 min.
Add the sweet potato cubes to the pan. Stir to coat in the cumin spiked oil and veg. Crumble the stock cube into the pan. Pour in 1 ltr boiling water. Simmer for 10 mins.
Drain and rinse the chickpeas. Tip them into the pan after the veg has simmered for 10 mins. Stir in the rainbow chard and half the lemon juice and zest. Simmer for 5 mins.
Taste and season the broth. Ladle into warm bowls and dollop on spoonfuls of labneh. Scatter over the mint leaves.