I went to Il Sedano Allegro with my wife three times when we were in Florence recently. The food we had was all quite good, and the owners are a charming, very friendly older couple. This really is a family-run restaurant.
I had a lot of good food here, but a few things stand out. My number one favorite was their onion soup. It was incredibly flavorful, and had a great texture. They cooked it with chunks of bread in the soup, and then broiled the bowl of soup in the oven, giving it a delicious crust on top. Amazing!
I also loved the pumpkin risotto. The pumpkin was pureed, and gave the dish an incredible rich creaminess, without any dairy. I also really liked their seitan and veggie kebobs. These were simple, but very tasty. I'd also highly recommend their crostini misti, a plate of toasted broad with different spreads, including red pepper, pumpkin, and eggplant.
The only reason I'm not giving this place five stars is that they didn't have any vegan desserts. That would have made the meals we had absolutely perfect.
Still, I highly recommend this place to any veg*n visiting Florence. The food is great, and the owners are wonderful.