1Combine Worcestershire, mustard, 1/4 cup of the oil, measured salt, and pepper in a shallow nonreactive dish and mix well.
2Cut the flank steak in half against the grain (that is, make your cuts perpendicular to the striated lines you see in the meat), so it will fit on the plate one half on top of the other. Pour the marinade over both pieces, rubbing it in with your fingers on all sides. Let the steak sit uncovered at room temperature for 20 minutes.
3Heat your frying pan over medium-high heat until hot, add the remaining 2 tablespoons of oil, and heat until the oil shimmers (you want it just on the verge of smoking). Brush off the excess marinade from one of the steak halves and add it to your pan (it shouldn’t be particularly wet when it goes in, otherwise the searing process will get slowed down).
4Cook your steak without budging it or moving it around, 3 to 4 minutes per side for medium rare, depending on how thick your steak is. It should be nicely browned. Flip the steak with a fork or tongs and cook the other side just the same. The internal temperature for medium rare should be 125°F to 130°F.
5Remove the steak from the pan and transfer it to a surface to rest. Let it sit for about 10 minutes (you can cook the other half while you’re waiting). Once it’s rested and the internal temperature has come down a few degrees, slice it thinly across the grain, ideally into strips no more than 1/2 inch thick (this will make it easier to chew). Season with additional salt and pepper. Repeat with the other half once it’s fully rested.
A balsamic vinaigrette doubles as a marinade here, giving flank steak enormous flavor while its marinated overnight. The dressing comes together quickly in a blender with garlic, rosemary, oregano and a touch of mustard.
A super hot pan, preheated under the broiler, will help to brown the underside of the steak while the broiler chars the top and cooks the meat through. The broccoli stems will become crisp-tender while the florets get deliciously frizzled.