Gallo Nero Restaurante

They advertise it as specialized medieval Tuscan food, and it's recommended in several sites I looked at. Situated on a beautiful street away from the hustle and bustle of Sienna, I would call for a reservation before going. I'll send you the website so that you can look their number up.
The food ends up being a better deal if you order off of the menus, or the "medieval tasting menu." It includes a drink and a dessert for around 25 euros and the portions are big. You'll eat dishes you've never tried before. The service is friendly and the place is cozy.