General tips
Pear is the secret. Korean bulgogi marinades use grated Asian pear (or kiwi in a pinch) for the enzymes that tenderize the beef from the inside. Skip it and you've got teriyaki. Use it and the meat practically melts. Marinate at least 4 h, ideally overnight.
Use ribeye, not sirloin. The marbling renders during the bath and the bulgogi marinade carries that fat back into the meat. Sirloin gets lean and tight at 130°–135°F. Slice the ribeye paper-thin against the grain after the bath, never before.
Cook at 130°–135°F for 2–4 h for the steak-like texture, or push to 140°–145°F for a more traditional well-done bulgogi. The lower temperature keeps the marinade from cooking out, so you taste the soy and sesame more clearly.
Sear in screaming-hot oil with sesame seeds added at the end. The whole point of bulgogi is the contrast between the silky interior and a deeply caramelized crust. A wok over the highest heat your stove can muster is the right tool.
Anti-tip: don't reheat bulgogi in the microwave. The marinade will cook out and the meat goes rubbery. If you have leftovers, throw them on a hot pan with a splash of water for 30 sec - just enough to warm through without re-cooking.
Tender tips
Asian pear tenderizes meat naturally - don't skip it!
Slice meat against the grain for maximum tenderness
Save marinade for drizzling over finished dish
Well-Done tips
This temp is great for meal prep - reheats well
Chop fine for kimbap (Korean sushi rolls)
Add to fried rice for quick weeknight dinner