In a small bowl, combine the rosemary, sage, thyme, salt and pepper and mix well. Pat the steak dry and coat it all over with the herb mix. Drizzle 1 tablespoon of the olive oil over one side of the steak to moisten the herb mixture and rub it gently into the mixture so it will adhere to the meat. Turn the steak over and repeat on the other side with another tablespoon of olive oil. Place the steak on a plate, cover with plastic wrap and let stand for 30-60 minutes to come to room temperature.
Meanwhile, pre-heat a gas grill or prepare a hot fire in a charcoal grill (use enough coals to keep the fire going for about 25 minutes).
Place the steak on the grill, cover the grill and cook until the meat is well charred on the first side, 10-12 minutes. Turn and cook for 10-12 minutes on the second side, or until the internal temperature registers 120°F. Fiorentina is traditionally served rare; for medium-rare, cook until the center registers 125°F. Transfer to a carving board and let rest, uncovered, for 10-15 minutes.
Meanwhile, in a large, heavy pot, heat the remaining 3 tablespoons olive oil over medium-high heat until very hot. Add the garlic and cook, stirring, just until golden. Add a few big handfuls of the spinach, sprinkle lightly with salt and cook, stirring or turning with tongs, just until wilted. Add the remaining spinach in batches, lightly seasoning each batch and stirring or turning just until wilted. Remove from the heat and add the lemon zest, juice and salt and pepper to taste.
Carve the fillet and the strip steak from the bone and slice the meat. Divide the steak among four plates, drizzle with olive oil and sprinkle with coarse salt. Place the spinach next to the steak and serve immediately.