Preheat the oven to 450 degrees F. Peel the beets and cut them into 1-inch chunks. Trim the Brussels sprouts. Cut the stems off the fennel bulbs. Then cut the bulbs in half, remove the cores, and cut the fennel into wedges. Finally cut the top off a whole head of garlic to expose the garlic cloves.
Place the potatoes, beets, and garlic head on a large rimmed baking sheet. Toss with oil, then salt and pepper generously. Spread the vegetables out in a single layer. Roast for 15 minutes.
Add the Brussels sprouts and fennel to the baking sheet. Toss to coat with the oil on the pan. Add the rosemary sprigs to the pan and place back in the oven for another 20-25 minutes, until the beets are fork-tender.
Meanwhile, drain the piquillo peppers and place them in the food processor. Add the walnuts, garlic, and all remaining ingredients for the sauce. Pulse until very smooth. Then taste, and salt and pepper as needed.
Pat the steaks dry on both sides. Then brush them lightly oil. Sprinkle with chopped rosemary, and salt and pepper generously on both sides.
If you have two ovens, preheat the second oven to broil and lower the top rack 6-7 inches below the broiler. If you are using one oven start the steaks after the vegetables have finished roasting.
Place a clean large rimmed baking sheet (13x18 inch) on the top rack to preheat. Once hot, carefully place the steaks on the hot baking pan and close the oven. Broil for 4 minutes per side, to caramelize. Then turn the oven down to 450 degrees F, and roast another 8-12 minutes (flipping once) until you reach your desired interior temperature. (120 degrees F for rare, 130 for medium-rare, 140 for medium.)
Allow the steaks to rest while you build the Bistecca alla Fiorentina platter. Transfer the roasted vegetables to a large serving platter. Squeeze the roasted garlic over the top. Scoop the Romesco Sauce into a bowl and add it to the platter. Place the steaks on top, and add lemon wedges to the platter. Serve warm.
Florentine Steaks are traditionally grilled, yet this recipe is adapted for winter holiday cooking. If you are willing to stand out in the cold, grill the steaks on high heat for 5 minutes per side. Then lower the heat to medium and grill another 3 minutes per side. Use this how to post to determine the doneness of your steaks. Although the steaks are gorgeous whole and uncut, you can stretch your steaks for a larger groups, by cutting the meat off the bones and slicing it before placing it on the platter.