Preheat the oven to 400 degrees . Cut 1/4 inch off the top of each garlic head and place cut side down on a sheet of foil. Sprinkle with 2 tbsp. EVOO and some salt. Wrap with foil and roast until tender, about 45 minutes. Let cool slightly, then separate the cloves and squeeze the garlic flesh into a bowl. Mash into a paste
Rub the steak with the garlic paste and place in a large baking dish. Add 13/4 cups balsamic vinegar, 2/3 cup EVOO, the smashed garlic and the rosemary sprigs. Refrigerate, turning occasionally, for 24 hours.
Preheat the oven to 350 degrees . In a large, ovenproof skillet, heat 1/4 cup EVOO over medium heat. Add the onions and cook until they start to brown on one side, about 3 minutes. Turn the onions, season with salt and pepper and place the skillet in the oven; roast until tender, about 10 minutes.
Meanwhile, heat a grill or grill pan to high heat. Remove the meat from the marinade and season well with salt and pepper. Grill, turning once, for 6 to 8 minutes for medium-rare; let sit for 5 minutes. Slice the steak on an angle 1/4 inch thick and arrange on a platter. Top with the figs and 2 tbsp. EVOO; season with salt and pepper. Arrange the onions around the steak.
In a large bowl, combine the radicchio, remaining EVOO and 11/2 tbsp. balsamic vinegar, and the red wine vinegar. Scatter the salad over the steak.