Slice steak thinly against the grain and marinate in soy sauce, brown sugar, ginger, and garlic for 5 minutes while rice finishes cooking.
Slice cucumber thinly and toss with rice vinegar and a pinch of salt to make quick pickles.
Heat a large skillet or wok over high heat and cook marinated beef in batches for 2 minutes per batch until caramelized, then return all to pan.
Divide rice among bowls, top with bulgogi beef and pickled cucumbers, and garnish with sesame seeds and green onion if desired.
Photo: Zhuojun Yu / Unsplash