In a slow cooker, combine the soy sauce, brown sugar, rice vinegar, sesame oil, garlic, ginger, and red pepper flakes. Stir until the sugar is dissolved.
Add the sliced flank steak to the slow cooker, making sure it is well coated with the sauce. Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the beef is tender.
Once cooked, shred the beef using two forks and mix it back into the sauce.
Warm the tortillas in a skillet or microwave until pliable.
Assemble the tacos by placing a generous amount of the shredded beef on each tortilla. Top with shredded cabbage, chopped green onions, and cilantro.
Serve with lime wedges on the side for squeezing over the tacos.