Chop the onion, cilantro, parsley, and celery leaves (set aside the stalks) in a food processor. Add 1/4 cup water and blend until finely chopped.

Sauté this mixture in olive oil, then add the finely diced celery stalks. Add the tomato paste, crushed tomatoes, and spices.

Pour in 8 1/2 cups water (2 liters). Add the lentils, cover, and simmer for 1 hour. Add the cooked chickpeas and simmer for another 20 minutes.

Mix 2 parts water with 1 part flour and 1 tablespoon tomato paste to make the thickener. Pour the thickener into the soup while stirring constantly. Add the vermicelli noodles and keep stirring to prevent sticking.

Once everything is cooked, adjust the seasoning with salt and pepper and sprinkle with cilantro. Serve the Harira with lemon wedges.