cmyr's recipe above is quite excellent, but a few notes for the sake of pedantry:

Lentil soup is perhaps even more popular in Turkey than in Greece. The Prophet reportedly said that lentils increase humility, which is why large banquets (and humble lunches) nearly always start with a bowl of lentil soup, known as mercimek çorbasi in Turkish.

The type of lentil used makes a big difference. Turks like red lentils, which cook relatively quickly and dissolve into a purée while doing so. The end result is a pleasing (and tasty!) bowl of thick, smooth yellow soup. As red lentils have a rather mild taste, go easy on the vinegar -- believe it or not, a single teaspoon is plenty for two quarts of soup.

You can also use green or brown lentils, but they will require a longer cooking time (I simmered mine 5 hours the last time around) and they will always retain some of their texture. While the end result is also quite tasty, it's definitely a vegetarian meal that is just brown gack, and probably not the type of thing you'd want to serve to guests.

You can also add various things to (brown) lentil soup to bulk it up: chick peas, tomato paste and bulgur are all popular. For spices, try basil, cayenne, coriander or mint. Lemon juice is often substituted for vinegar.

For any lentil soup, some fresh bread on the side is a nearly obligatory accompaniment. If reheating the soup, add some water to re-liquefy it.