Pat the flat iron steaks dry with paper towels. Trim any excess silver skin or large fat deposits. Lightly score the surface of the steaks with shallow 1/8-inch cuts in a crosshatch; this helps the marinade penetrate and prevents curling.

Prepare marinade

In a small bowl combine the minced garlic, chopped rosemary, thyme, olive oil, Worcestershire sauce, red wine vinegar, kosher salt, and freshly ground black pepper. Whisk until well combined.

Marinate

Place steaks in a shallow dish or a resealable plastic bag. Pour the marinade over the steaks, turning to coat both sides evenly. Massage marinade into the meat for 30 seconds. Refrigerate and marinate for at least 30 minutes and up to 4 hours. Remove steaks from the refrigerator 30 minutes before cooking to come to room temperature.

Preheat pan or grill

Heat a heavy cast-iron skillet or grill to high heat (the pan should be very hot — about 500°F/260°C surface temperature if using an infrared thermometer). Lightly oil the grate or brush a thin layer of oil on the steaks if cooking in a pan to prevent sticking.

Season before searing

If any marinade pools on the surface, blot excess with a paper towel. Sprinkle a light additional pinch of kosher salt and freshly ground pepper on both sides immediately before searing.

Sear the steaks

Place steaks on the hot grill or skillet. Sear without moving for 3 to 4 minutes to develop a deep brown crust. Flip and sear the other side for another 3 to 4 minutes for medium-rare (internal temp 125–130°F / 52–54°C). For medium, cook 4 to 5 minutes per side (135°F / 57°C). Use an instant-read thermometer to check doneness.

Finishing & Serving

Butter baste

During the last 1 minute of cooking, reduce heat slightly (if using a pan) and add the unsalted butter to the skillet. Tilt the pan and spoon the melted butter over the steaks repeatedly (baste) for about 30–60 seconds. If grilling, place butter on top of the steaks to melt over them just before removing from heat.

Rest

Transfer steaks to a cutting board and tent loosely with foil. Rest for 8 to 10 minutes to allow juices to redistribute; the internal temperature will climb a few degrees during this time.

Slice and finish

Slice the steak thinly against the grain to maximize tenderness. Squeeze the fresh lemon juice over the slices, sprinkle with chopped parsley and optional flaky sea salt to taste. Serve immediately.