In a large plastic bag, combine the soy sauce and mirin. Add the salmon, coating it well with the marinade, and seal the bag. Let the salmon marinate, chopped, for at least 1 hour and up to 6 hours.
Remove the salmon from the bag and discard the marinade. Spread the sesame seeds on a plate and season the salmon with salt and pepper, then coat both sides of each steak with the seeds, patting them on to make sure they stick.
Preheat the grill, or broiler with the rack in the top position.
Brush the grill with oil and grill the salmon ( or broil it on a rack in a baking pan) for 6 minutes on each side or until just cooked through.