Combine the soy sauce, sesame oil, water, mirin, garlic, and sugar together in a large zip lock bag. Slice the flank steak, against the grain, into small bite size pieces. Place the meat into the marinade and refrigerator for 2-24 hours.
Combine the soy sauce, lime juice, sesame oil, and sugar together in a small bowl to make the salad dressing. Whisk until well combined then set aside to let flavors mingle.
Heat a grill pan over medium high heat. Coat with cooking spray. Once the pan is hot, add the flank steak, and cook for 2-3 minutes, stirring often until the meat is cooked to your desired degree of doneness. Remove from the grill pan and set aside. Pour some of the marinade into the grill pan and cook on high for 2-3 minutes until it has boiled and reduced a bit. Pour the reduced marinade onto the cooked beef and toss to coat evenly.
Chop the romaine lettuce and the napa cabbage finely. Toss them together with the green onion, shredded carrots, and cilantro. Drizzle the salad dressing on the salad, to taste. Toss to coat evenly.
Wrap the flour tortillas in a damp towel and cook in the microwave for 30 seconds to warm them up. Place some of the meat into the tortilla followed by the salad. Top with some sriracha sauce. Serve the tacos with lime and eat immediately. Enjoy!