In a large pan, heat the oil over a moderate heat. When hot, add the bay leaf, onion, celery and garlic. Fry gently for 5 minutes until the onion and celery are soft.
Add the lentils and stir for 1 minute before pouring in the stock. Add the tarhana and season with pepper and about 1 tsp salt (you can adjust later). Stir well and bring to a simmer.
Turn down the heat to low and simmer the lentils gently for 30-35 minutes. Stir often as the tarnaha can sometimes stick to the bottom of the pan. If you soup looks too thick, add more stock or water to thin out.Once the lentils are tender, remove from the heat. Adjust the salt levels if you need then stir in ¾ of the herbs.
Ladle into serving bowls and scatter generously with more herbs. If using, scatter over some feta cheese and serve hot.