In a large, shallow dish, combine the soy sauce with the sugar, white wine, chopped garlic, toasted sesame oil and crushed red pepper, stirring to dissolve the sugar. Add the sliced flank steak and coat thoroughly in the marinade. Cover and refrigerate the steak for at least 4 hours or overnight.
Light a grill or heat a griddle. Rub the scallions all over with vegetable oil and grill them over high heat, turning once, until the scallions are just softened, about 2 minutes. Season with salt.
Working in batches, grill the steak over high heat until the slices are richly browned and medium-rare, about 30 seconds per side. Transfer the steak to a serving platter and serve with the grilled scallions and steamed rice.