If you have time, remove steak from the refrigerator 30 minutes prior to cooking. (This is optional, but recommended for even cooking.)

Pat both sides of the steak dry with a paper towel. Season both sides with salt and pepper.

Stovetop Method: Preheat a large cast iron skillet over medium-high heat and add oil. Once hot and shimmering, place the flank steak in the pan. Sear for 2 minutes per side for rare, 2.5-3 minutes per side for medium rare, 3-3.5 minutes per side for medium, 3.5-4 minutes per side for medium well, or 4-4.5 minutes per side for medium well.Grill Method: Preheat the grill to high heat (~450˚F). Lightly oil the grill grates. Once heated, place the steak on grill. Close the lid and cook for 1 minute, then open and flip the steak over. Close the grill and cook for another 1-2 minutes (the steak should have a brown sear on both sides). Reduce heat to medium. Keep the lid closed (except to flip again once) and cook for an additional 2.5-3 minutes per side for rare, 3.5-4 minutes per side for medium rare, 4.5-5 minutes per side for medium, 5.5-6 minutes per side for medium well, or 6-7 minutes per side for well done.For both methods, cook times may vary depending on the thickness of the steak, so use a meat thermometer to check: 120 degrees F (49 degrees C) for rare, 130 degrees F (54 degrees C) for medium rare, 140 degrees F (60 degrees C) for medium, 150 degrees F (66 degrees C) for medium well, or 160 degrees F (71 degrees C) for medium well.

Remove from heat and transfer to a plate. Cover with aluminum foil and let rest for 10 minutes. (The internal temperature will rise an additional 5 degrees.)

Meanwhile, make the chimichurri sauce: Combine the olive oil, vinegar, parsley, garlic, oregano, salt, and crushed red pepper in a glass jar. Shake until emulsified. (Adjust sea salt and red pepper to taste, if needed.)

Slice the steak into thin strips, against the grain. Serve with chimichurri sauce.