First, slice the pork butt into strips as thin as possible and place in a large bowl.
In another bowl, combine chopped onion, minced garlic, grated ginger, soy sauce, rice wine vinegar, sesame oil, sambal oelek, sugar and black pepper. Mix to combine.
Pour sauce over pork slices and mix to fully coat the pork. Cover bowl with plastic wrap and let marinade for at least 30 minutes - up to 8 hours. If you are in a hurry, 30 minutes will do. But, the longer you marinade the pork, the more the flavors will sink into the pork. If you are marinating pork for longer than 30 minutes, place in the fridge and be sure to remove the pork from the fridge about 20 minutes before cooking.
Heat a cast iron skillet or non stick skillet to medium-high heat and add 2 tablespoons of oil. Now, carefully add the pork to the hot skillet, and sear, stirring and turning pork, to ensure it is not burning. Sear until pork is cooked through.
Sprinkle pork with sesame seeds and green onions. Serve with steamed white rice. Enjoy!