In a large bowl, combine tamari or soy sauce, maple syrup, coconut sugar, garlic, ginger, rice vinegar, fish sauce, sesame oil, gochugaru, and black pepper. Mix these ingredients well until the sugar is dissolved.
Add the thinly sliced beef sirloin to the marinade. Make sure each piece is generously coated. Cover the bowl and let it sit for at least 30 minutes. If time allows, an hour or even overnight in the fridge will allow the flavors to meld beautifully.
In a large skillet or grill pan over medium-high heat, add a splash of oil. Once hot, add the marinated beef, making sure not to overcrowd the pan. You want that lovely sear! Cook for about 4-5 minutes.
After the beef is nearly cooked, toss in the sliced yellow onion. This adds sweetness and balances the savory profile. Cook for an additional 2 minutes or until the onions are tender.
Once cooked through, remove from heat. Sprinkle the beef bulgogi generously with toasted sesame seeds and chopped green onions. Serve hot over steamed rice or in lettuce wraps.
