Place a large cast iron skillet over medium-high heat. (You can also cook this recipe on the grill or use a grill pan.) Do this before you start prepping. The skillet should be really hot when you start 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. Be sure to make the second set of holes 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 each side of the steak with half of the seasoning mixture.
Add 1 tablespoon each 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.
Remove the steak from the pan and allow it to rest for 10 minutes, 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 salsa and then the wilted scallions. Serve family-style.
If corn is in season, I'll roast the corn on the grill and use that for the salsa. 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.