Pat the flat iron steak dry with paper towels. Trim any large pieces of silver skin or excess fat, leaving a thin cap for flavor. If thicker than 1 inch, lightly score the surface or use a meat mallet to gently pound to an even thickness for even cooking.

Make marinade

In a shallow dish or resealable bag, combine 2 tbsp olive oil, 1 tbsp soy sauce, 1 tsp Worcestershire sauce, minced garlic, 1 tsp kosher salt, 1 tsp freshly ground black pepper, 1 tsp smoked paprika, and chopped rosemary (if using). Whisk or shake to emulsify.

Marinate

Place the steak in the marinade, turning to coat thoroughly. Let sit at room temperature for 20–30 minutes if short on time, or refrigerate for 2–6 hours for deeper flavor. If refrigerated, remove and come to room temperature for 20–30 minutes before cooking.

Preheat pan or grill

If using a cast-iron skillet, place it over medium-high heat and let it get very hot (about 5–7 minutes). If grilling, preheat to high (about 450–500°F / 230–260°C). Brush a little oil on the cooking surface to prevent sticking.

Sear the steak

Remove steak from marinade and let excess drip off. Place steak on the hot skillet or grill. For a steak about 3/4–1 inch thick, cook 3–4 minutes per side for medium-rare (internal temp 130–135°F / 54–57°C). Adjust time ±1–2 minutes per side for thicker or thinner cuts and desired doneness. Resist flipping more than once.

Finish with butter (optional)

During the last 1 minute of cooking in the skillet, add 1 tbsp butter to the pan and tilt the pan, spooning melted butter over the steak to baste and add richness. If grilling, remove steak briefly to a cooler part of the grill and brush with melted butter.

Check temperature

Use an instant-read thermometer inserted into the thickest part: 120–125°F for rare, 130–135°F for medium-rare, 140–145°F for medium. Remember the steak will rise about 3–5°F while resting.

Resting & Serving

Rest the steak

Transfer steak to a cutting board and tent loosely with foil. Rest 8–10 minutes to redistribute juices. Do not cut immediately or juices will run out.

Slice and finish

Against the grain, slice the flat iron steak thinly (1/4-inch slices or to preference). Sprinkle with chopped fresh parsley and a pinch of additional salt or pepper if desired. Serve with lemon wedges on the side for brightening, if using.

Serve

Arrange slices on a warm plate and spoon any collected resting juices over the top. Serve immediately.