Season flank steak with salt and pepper on each side (use more salt and pepper as desired). Set aside.
In a cast iron (or stainless steel) pan over medium-high heat, add oil. When hot and shimmering, add flank steak with tongs and allow to sear for 2 and 1/2 minutes on the first side. When flipping your steak with tongs, if it is seared properly, it will pull away easily from the pan.
Flip steak and sear for an additional 2 and 1/2 minutes. For a 1 and 1/4" flank steak (like pictured) this will give you a medium-rare flank steak. Add additional time as needed for a thicker or more well-done steak.
Cooking times for a steak will vary based on the thickness, size, and shape of your steak. For best success, I suggest a meat thermometer. You'll want to remove your steak from the pan when the temperature reaches 5 degrees below your desired doneness. The steak will continue to cook slightly while it is resting. - Rare: 125 degrees (Fahrenheit)
- Medium-rare: 135 degrees
- Medium: 145 degrees
- Medium-well: 150 degrees
- Well done: 160 degrees.
Once your steak is cooked to your liking, remove from the pan, tent with foil, and allow to rest for 10 minutes. Slice steak thinly and against the grain.
Meanwhile, turn the heat to medium-low and return pan to heat. Add butter to pan and melt. Add garlic to pan and cook for up to 1 minute, or until garlic begins to brown (garlic will brown quickly).
Remove from heat and pour garlic butter over flank steak. Serve and enjoy.
I highly suggest using a meat thermometer if you are not familiar or comfortable with pan-searing steaks. Due to the variance in thickness, temperatures of the stove top, size, and shape of your steaks, the cooking times may vary.