Pat the steak dry with paper towels. Cover with salt on all sides. Add black pepper to taste, along with the oregano and cumin.
Leave it uncovered in your fridge overnight. As it sits, the salt extracts the juices from the steak and flavors them. The steak will then re-absorb this brine, which results in deep seasoning. Pat the steak dry again before you brush with oil. If you don't have that kind of time, leave the steak out on your counter for 30 minutes to bring it up to temp, dry it thoroughly, brush with olive oil and add the salt and other spices just before it hits the pan.
Place a cast-iron pan on your stovetop using high heat. Your pan is ready when a drop of water hits it and immediately evaporates. The pan will start to smoke. Add a good ¼ inch coat of high-smoke-point oil, like grapeseed.
Use your tongs to hold the steak on its side and sear the borders, making sure to brown any fat thoroughly, about 1 minute.
Sear the steak on one side for 2 minutes. Flip the steak onto its unseared side.
Add 2 tablespoons of butter to the pan along with the smashed garlic clove. Tilt the pan so the butter collects by the handle. Use your spoon to baste the steak as it comes up to temp.
Remove the steak from the pan when it reaches 5 degrees below your desired temperature, to makeup for carryover cooking. For rare take it out at 120°, medium-rare 130°, medium 140°, and well-done 150°. Use a digital thermometer for best results. Let the steak rest on a rack over a sheet pan.
While your steak rests, remove the garlic and all but 2 tablespoons of drippings from the pan. Lower the heat to medium. Sautee the onions in the drippings. Add a pinch of salt. Once the onions have started to caramelize add the minced garlic. Sautee briefly until the garlic blooms.
Deglaze the pan with the vinegar and chicken stock. Bring it to a boil and cook it down until the sauce has thickened. Take it off the heat and drop in the butter to thicken the sauce even more and give it a glossy finish. Drizzle the steak with the onions, garnish with cilantro, and serve with a side of rice and beans.