Combine all the marinade ingredients in a blender or food processor and blend until perfectly smooth. Set aside.
Place the beef in a large bowl and pour the marinade over the top. Gently massage the meat so every slice is coated.
Drizzle in the sesame oil and toss to combine.
Cover with plastic wrap (or transfer to an airtight container) and refrigerate for at least 4 hours, preferably overnight.
Heat a large skillet over medium-high heat and add the neutral oil.
Add the marinated beef along with the onion, green onions, and carrot. Stir-fry for 3–5 minutes, or until the meat is just cooked through. Sprinkle in the sesame seeds and give everything a quick toss.
Serve hot over steamed rice, garnished with extra green onions and sesame seeds. Add any of your favorite Korean side dishes on the side.
Notes
Pop the beef in the freezer for about 20 minutes first; slightly firm meat is much easier to slice paper-thin.