For the marinade:
1/2 cup low-sodium soy sauce
1/4 cup pineapple juice
2 tablespoons rice vinegar
2 tablespoons toasted sesame oil
2 green onions, white and light green parts only, minced (reserve ends for garnish)
2 large cloves garlic, minced
1 tablespoon packed brown sugar
1 tablespoon toasted sesame seeds, plus more for garnish
1 teaspoon grated ginger
1 pound flank steak, cut into 1/8-inch slices against the grain (see Recipe Notes)
For the finished dish:
Neutral cooking oil, such as canola, for grilling
Cooked white rice, to serve (see Recipe Note)
Combine the ingredients for the marinade in a mixing bowl and transfer to a sealable plastic bag. Add the beef slices, press any excess air out of the bag, and seal. Refrigerate for a minimum of 30 minutes, or overnight.
Heat a large griddle or grill pan over medium-high heat. Using a silicone brush, generously brush oil on the pan and wait until it is sizzling. Working in batches if necessary, sear the meat until deep-brown grill marks form on one side. Flip and continue cooking until caramelized and cooked through, 30 seconds to a minute. Garnish with green onions and toasted sesame seeds and serve with cooked white rice.