Bistecca Fiorentina - Siena or Rome
For the real thing, you'll want to look for this in Siena, not Rome. Even better would be to venture into the hill towns north of Siena, toward Fierenza and find a local trattoria off the beaten path.
A regional dish like Bistecca Fiorentina is specific to it's local and its preparation is typically a classic recipe using a specific cut of beef from a specific herd of cattle with the dish being dressed with a specific olive oil, etc. The farther afield one gets from the origin of the dish, (sometimes only a few kilometers) the more it becomes someone else's take/twist on the classic, or not served at all.