On a shallow plate, grate the garlic and mix it with the EVOO. Place steaks in the oil, flipping them around a few times, the cover and marinate for at least 30 minutes.
Heat a large, cast-iron skillet over medium-high heat. Shake excess oil from the steak, season both sides with salt and pepper, and cook until desired doneness, about 4 minutes on each side for medium rare. Remove from the skillet and let rest for about 5 minutes.
While the meat is resting, make the gravy: Add a little of the reserved garlic and oil marinade to the skillet then stir in the flour and cook 1 minute. Whisk the balsamic and beef stock into the flour mixture, season with salt and ground black pepper and cook until thickened.
Slice the steak and top with the balsamic gravy and the baby spinach or arugula. Serve with Sage-Butter Gnocchi alongside.