• Optional: Leave flank steak at room temperature for 30 minutes before you start the prep.

Prep flank steak

• Use the tip of a sharp knife to poke several small slits all over the meat. Slits should not be deep enough to go all the way through, just about half way.
• Rub kosher salt on one side of the steak. Continue with freshly ground black pepper, dijon mustard, and 2 tablespoon softened butter over each side of the steak. I like to do this on the cutting board.

Cook

• Heat cast iron skillet to high heat. Use tongs to place the flank steak in the hot skillet. Cook steak for 2 to 3 minutes per side, being sure to get a good sear on each side. It's ok to flip the steak over while searing. You'll be able to see how the crust is forming.
• Turn the heat off. Let the steak finish cooking in the residual heat of the pan for about 5 to 10 minutes. If you didn't use a cast iron skillet, you might want to keep your frying pan on low heat.

Check for doneness

• Insert a meat thermometer into the thickest part of the steak. Internal temperature should read between 120 degrees F to 130 degrees F (very rare to medium rare). This is the best temperature range for tender, juicy meat. If you want to cook the steak a little more, put it back in the pan and heat over medium-high heat for a few minutes.

Rest

• Rest meat on a wooden cutting board or a wire cooling rack (like one you use for cookies) for about 10 minutes. You can tent with aluminum foil, just be sure you are just loosely covering the steak and not trapping any steam inside. This is optional.

Make mushroom pan sauce

• While the pan-seared flank steak rests, heat the skillet back up to high heat. Add the sliced mushrooms and the water to deglaze the pan. If there's any juices from the resting steak, you can add that to the pan to make the sauce. Season with a little salt and pepper and cook until mushrooms are tender and a thin sauce forms. Add the remaining 1 tablespoon of butter and stir until melted.

Slice steak

• It's important to cut the steak properly so that it's not chewy. Look at the direction the muscle fibers (the grain) are going. You want to cut at an angle, against the grain, into thin slices. This will help tenderize the meat.
• Pour mushroom sauce over the cut flank steak and serve with fresh herbs or sliced scallions.