One of the best meals to have in Tehran is at Moslem Restaurant in the Tehran Grand Bazaar. Just from the lunch line you can tell this place is amazing! It was my first experience dining at Moslem and it was quite the learning experience. First you grab a ticket to get in line - the line moves pretty quickly. Then once you go upstairs and get inside the staff hands you a tray and you get your sides which include yogurt, olives, salad, drinks etc. Afterward you are asked how many people are in your group in order to be seated. Since we were a group of 11 people they sent us upstairs to the top level. Once upstairs one person goes in line to order for the group while another server finds you a table and seats your party. You place your order for the famous tahchin and pay the bill. Then you wait till they bring out your delicious meal. They have many delicious items on the menu, the tahchin being the most popular, and the portions are out of the world. One order could easily feed 2-3 people.
As a foodie this was one of the best experiences I had in Tehran!