Rick's DVDs and books on Tuscany talk about slow food but reference specific places. Found some sights on the web and will also ask for recomendations at the agroturismos when I get there but was wondering is anyone had tried farms or establishments in the Siena area they thought were worth recommending?
I would not spend any time trying to find a specific restaurant. The food in Tuscany is all excellant. Your hosts where you are staying will be by far your bests bets for recomendations. Have been in the area many times and never had a meal less than excellant, and the small places frequented by the locals will be very reasonable pricewise - sometimes very inexpensive.
You really can't go wrong anywhere in Tuscany, just about any agriturismo would be great, and would adhere to the "slow food" principles. Also you never know where the best places might pop up. We once stayed in this so-so hotel outside of Pisa. The hotel was nothing special, but the food--we still talk about it almost 6 years later.