Step 1Heat the oven to 400 degrees. In a small bowl, stir together the balsamic vinegar and the garlic. Salt the steaks with one-half teaspoon salt on each side and then brush lightly with the balsamic vinegar mixture. Set aside for at least 30 minutes.
Step 2In a small saucepan over medium-low heat, heat the caramelized onions with the rosemary, stirring occasionally until warmed.
Step 3Heat a cast-iron skillet or griddle pan over medium-high heat until it is very hot (a drop of water should skitter across the surface). Brush both sides of the steak once more with the vinegar mixture and then sear in the cast-iron pan until well-browned on both sides (about 3 minutes each). Sear the sides as well. Once the steaks are browned, put the pan in the oven and cook, turning once, until the steaks are medium-rare (about 125 degrees), 10 to 12 minutes total cooking time.
Step 4Remove the steaks from the oven and transfer them to a carving board. Rest them for 5 minutes to let the juices re-distribute through the meat. While the steaks are resting, add the sherry vinegar to the onions and season to taste with more salt or vinegar if necessary.
Step 5When the steaks are ready, spread a very thin layer of the caramelized onion marmalade over top, and then carve the steaks and serve them with the remaining marmalade on the side.