Preheat oven to 400°F.
To make the balsamic reduction, place the vinegar, maple syrup, garlic and mustard in a small saucepan. Simmer, stirring occasionally, until thickened and syrupy, 10-12 minutes. Remove from heat.
Sprinkle the top of the steak with a generous pinch of salt and pepper.
Heat the oil in a large cast iron skillet over medium high heat on the stove. When the oil is hot, add the steak, seasoned side down. Season the second side with salt and pepper.
Sear the steak without moving it until browned, about 4 minutes. Flip and cook another 2-3 minutes on the second side. Transfer the skillet to the oven.
Cook to desired doneness, flipping the steak halfway through cooking.
Let the steak rest for 7-10 minutes. Sprinkle a pinch of salt on top, slice, and serve with the balsamic reduction.