Clean any silver skin left on the flank steak by running your knife carefully under the thin milky white membranes on the meat. Take caution to not cut too deeply. Discard the membrane.
Use a tenderizing mallet, work the meat in a zigzag pattern hitting all the spots till there are tiny punctures evenly throughout the skirt steak. Flip and repeat.
If you don’t have a tenderizer, use a fork to puncture holes in a zigzag pattern and repeat the process as you would with a mallet. Set the skirt steak aside in a bowl or Ziploc bags.
Make the marinade: Combine the onion, garlic cloves, ginger coins, brown sugar, soy sauce, Sprite, mirin, black pepper, sesame oil, black pepper, and jalapeno pepper together in a blender, secure the lid, and blend until smooth and combined. Transfer to a large bowl and add the sesame seeds and scallions.
Add the steak to the bowl, cover, and refrigerate overnight.
When you’re ready to cook, heat the oven to 350°F. Remove meat from bowl and discard marinade.
Place a cast-iron skillet over medium-high heat. When hot, add the oil. When the oil is shimmering and hot, add the steak and cook for 3 minutes. Flip and cook for 2 minutes.
Place skillet in the oven and cook for another 2 to 5 minutes. Remove from oven and rest for 3 minutes before slicing.