Heat a large skillet or soup pot over medium-low heat, and add 2 tbsp olive oil. Saute the onion, celery, celery leaves, and garlic, covered, until soft and translucent, about 15 minutes.
Add the rosemary, chickpeas and stock and bring to a gentle boil. Simmer for 20 minutes until the chickpeas are soft. Remove half the chickpeas with a slotted spoon, and purée the remaining soup in a blender, through a food mill, or with an immersion blender. Return the purée and reserved chickpeas to the pot, and add the pasta.
Cook until the pasta is tender, adding boiling water if necessary to loosen. Season to taste with salt and pepper and the lemon juice. Serve with parsley or basil leaves and a drizzle of olive oil.