Place a large cast iron skillet over medium-high heat. Do this before you start prepping. The skillet should be really hot when you start cooking. (see notes for grill cooking)
Trim the flank steak of any excess fat and tough connective tissue over the steak's surface. Grab a paring knife and insert the knife at an angle, almost all the way through the steak, to create multiple holes all over the surface. Flip the steak over and repeat the process on the other side - making sure the second set of holes is parallel to the first set so you don't create gaping holes in the steak.
In a small bowl, stir together the salt, pepper, ground mustard, and onion powder. Season both sides of the steak with the seasoning mixture. Set aside.
Add 1 tablespoon each of olive oil and butter to the skillet. Once the butter melts, add the whole green onions. Cook for 30 seconds to 1 minute per side until the greens have wilted and are slightly charred. Remove from skillet and set aside.
Add the remaining olive oil and butter to the same skillet. Sear the flank steak for 2 minutes on each side until a nice brown crust forms. Turn off the heat and let the steak hang out in the skillet for another 5 minutes, flipping halfway through. (The steak will be cooked medium.)
Remove the steak from the pan and allow it to rest for 10 minutes on a cutting board loosely covered with aluminum foil.
In a medium mixing bowl, combine the corn, jalapeno, lime zest, lime juice, ginger, and Sriracha until combined. Season with salt and pepper to taste.
Once the steak has rested, slice it into thin strips against the grain. Place the steak slices on a large platter. Top with the salsa and then the wilted scallions. Serve warm.
Notes
If corn is in season, I'll grill the corn and use that for the salsa. (1 1/2 cup total) You can also use frozen corn that's been steamed and cooled.
If you want to add some heat to your salsa, increase the amount of Sriracha to taste and/or leave in the jalapeno seeds.