Preheat your cooking device: heat the grill to 420°F (215°C) or preheat the oven to 400°F (200°C) if finishing steaks in the oven; if using the oven method, preheat a cast-iron skillet on the stovetop as well.
Prepare the steaks: pat the ribeyes dry, then season both sides generously with the 1/2 teaspoon salt, 1/2 teaspoon pepper, 2 tablespoons onion flakes, and 4 tablespoons minced fresh rosemary; drizzle and rub with 1/4 cup olive oil to coat both sides.
Make the rosemary-parmesan butter: in a bowl, combine 1/3 cup room-temperature unsalted butter, 1 tablespoon minced fresh rosemary, 1 tablespoon onion flakes, 1/4 teaspoon salt, 1/4 teaspoon pepper, and 1/4 cup freshly grated Parmesan until smooth; set aside.
Grill method: place the steaks on the preheated grill with the lid closed and cook 7–9 minutes on the first side, then flip and cook another 7–9 minutes, adjusting time for desired doneness.
Oven (pan-to-oven) method: sear the seasoned steaks in the hot cast-iron skillet 2–3 minutes per side on the stovetop, then transfer the skillet to the 400°F (200°C) oven and roast 10–12 minutes, or until the desired doneness is reached.
Check doneness with a meat thermometer and remove the steaks when they are about 3°F (≈2°C) below your target final temperature.
Finish and rest: remove steaks from heat, dollop the rosemary-parmesan butter on each hot steak, and let rest for 7–10 minutes before serving.