Combine soy sauce, vinegar, sesame oil, wine, brown sugar, mirin, ginger, and red pepper flakes. Mix well. Transfer ¼ cup of mixture to a large saucepan. Place fish in dish and spoon marinade over fish. Let stand 30 minutes.
Heat grill to medium-high direct heat. Oil the grill rack and grill fish for 3 minutes on each side, or until cooked through. Remove from heat, cover with foil, and set aside.
Meanwhile, heat the remaining marinade over medium heat until hot. Add half the spinach and cook 2 minutes until wilted. Add the remaining spinach and cook until wilted.
Transfer spinach to 4 serving plates, top with tilapia. Sprinkle with sesame seeds and serve. Steamed Jasmine rice makes a perfect side dish.