Scoop the flesh out of the sweet potato. Put the flesh, chickpeas (reserving a few to garnish), garlic and tahini into a food processor and blitz until well-combined.
Using a pestle and mortar, pound the cumin and coriander seeds until finely ground. Add to the processor (reserving a pinch to garnish) with the lemon zest and juice, 1 tsp salt and about 3 tbsp olive oil. Blitz again until smooth. If it is still quite stiff, add more olive oil and blitz until you have a soft, smooth puree. Season to taste.
Serve with the reserved chickpeas, spices and parsley sprinkled on top and some olive oil drizzled over.