Combine the sesame oil and salt in a small bowl and set aside.
Thinly slice the scallion greens into 3-inch-long julienne. Stir together the soy sauce and rice vinegar in a small bowl. Add the scallions, toss and set aside.
Lightly oil the grill and rub the grates with one of the onion rounds. Place the onions on the grill and grill until slightly charred and softened, 3 to 4 minutes per side. Move the onions off to the side to keep warm. Grill the rib-eye slices until browned and cooked to the desired doneness, about 2 minutes per side.
To eat, dip a piece or two of the rib-eye into the seasoned sesame oil and then place in a lettuce leaf along with some grilled onions. Top with some marinated scallions and consume like a taco.