Combine soy sauce, garlic powder, salt, and pepper in a shallow bowl and marinate flank steak for 2 hours.

Preheat an outdoor grill for high heat and lightly oil the grate.

Remove flank steak from marinade and grill until slightly crusty on the outside but only medium warm on the inside, about 8 minutes per side. An instant-read thermometer inserted into the center should read 130 degrees F (54 degrees C). Remove, cover, and let rest for 10 to 15 minutes.

Melt butter in a saucepan over medium-low heat and cook green onions until tender, about 5 minutes. Add red wine and increase heat. Bring sauce to a simmer and cook for 5 to 10 minutes until all the alcohol has evaporated. Season with salt and pepper.

Slice steak as thinly as possible and arrange on a platter. Add any juices from the steak to the sauce and drizzle sauce over steak, reserving some of the sauce to serve at the table.