Preparation

• To tenderize the steaks, lay them flat on a cutting board. Tap the steaks with the back edge of a kitchen knife (the edge opposite the blade) to dig notches into the meat. Turn the knife so the flat side is facing the fillets. Pound the meat with the knife’s flat side about 6 to 8 times on each side of the steak to flatten the meat. Season both sides of the meat with salt and pepper.
• Melt 2 Tbs. of the butter in a skillet over low heat. Add the onion and garlic and cook, stirring frequently, for about 8 minutes, until the onions are translucent and caramelized and have developed their sweetness. Increase the heat to medium. Add the mirin and soy sauce and cook, stirring constantly, for about 1 minute, to cook down the liquid. Add the vinegar and cook, stirring constantly, for 30 seconds more. Transfer to a bowl and set aside.
• Melt the remaining 1 Tbs. butter and the oil in a large skillet over medium-high heat. When the butter has melted and started to brown, cook the steaks in batches. Lay the steaks in the skillet, and cook for about 4 minutes, turning once (for medium rare). Transfer the steaks to a cutting board and allow them to rest for 1 minute before serving.
• Return the skillet to medium-high heat. Add the onions and cook, stirring constantly, for about 10 seconds, so the onions absorb the steaks juices remaining in the skillet.
• Top the steaks with the onion mixture, garnish with the parsley, and serve immediately.