Chianina Beef in Florence

I've never tried that but I assume you're referring to Bistecca alla Fiorentina? From what I've seen during travels, those are usually a very large portion, so you'll need to have a good appetite. I found a few possibilities..... > Buca Lapi (1R Via del Trebbio) > I' Cche' c'e' c'e' (Via dei Magalotti, 11) - that item is listed on the Menu at €38) I have dined at the latter restaurant, and would highly recommend it! I'm sure Roberto will be along shortly, and will have lots of good suggestions. Buon Appettito!

Trattoria Sostanza. They also have a butter chicken that is said to be very good.

It is served at many restaurants. I used one of Rick's recommended places. (It was on the piazza down the hill behind the Il Campo) It was a very good steak. It is designed to be split between two people. And is served by the Kilo. Ours was 49 Euro per kilo(2.2 lbs). It is a T-Bone cut of meat so you are paying for the bone too. So splitting it made for a normal sized steak dinner for two. Again it was VERY good.
But the bill came for the actual weight of meat which was a little more than the posted ONE kilo price. Must have been a 1.2 kilo piece of meat before it was cooked. And my Rick Steves Best of Italy Tour travel buddy who split it with me picked up the bone and finhed off the meat that was left on the bone. He beat me to it. It was really good and worth the cost. IMO.

Mario's is excellent! Don't even think about telling them you want it medium anywhere in Florence. You will get it how they cook it, and it will be delicious. Order two sides and a bottle of wine. The highlight of Florentine food. We usually order the spinach and a roasted potato. Roberto is right. The Florentine's are not at all shy about shaving every last bit off the bone and eating it! When in Florence do as the Florentines do! There's a great lunch counter place on Via Gibbolina - near the Santa Croce church. It's only open from 11 - 2;30. Picnic tables inside, and their Caldo Bar is incredible they may have soup may have rabbit, beef and pork, always chicken, I think the name of it is Fratellini. Many side dishes, delicious beans, etc., and many vegetables. Also Panini. Eat a BIG lunch there, and a small pizza for dinner! Other than the Florentine Bisteca, my favorite meal in Florence.
Very worth walking a block or two if you happen to be near the Santa Croce church, which is one of my favorites! There's a Star of David over the main entrance honoring the Jewish architect who is buried under the stairs at the entrance. Take a look at the bones of DaVinci, Michelangelo, and way too many others to even name. The crypts have the most beautiful sculptures on top of them!

Perseus is one of the best. Located on Viale don Minzoni near piazza della Liberta'.

Ken, I too loved my time at Mario's, near Mercado Centrale. The atmosphere is fun (shared common tables), they serve absolutely delicious food, have reasonable prices and it is a great way to meet locals and tourists alike. I met some wonderful people from South Africa there; we shared the table and enjoyed a real fun afternoon together. But, get in line early, as Marios is a popular dining experience and they get quite busy. Ciao!