In a zip-lock bag, combine the soy sauce, chopped onions, lemon juice, sesame oil, and fresh ginger. Shake everything around in the bag until it's well combined.
Add the tuna filets to the bag, and shake until covered completely. Remove all air from the bag and leave to marinade for 20 minutes in the fridge.
Remove the filets from the bag and reserve the mixture for later.
Coat the filets in sesame seeds on a plate, until the surface is covered.
Add the sauce to a small saucepan, whisk in the cornstarch and heat until thickened.
Fry the steaks in a pan for around 3 minutes per side.
Slice into strips and serve on a long rectangular plate
Drizzle over the sauce and enjoy!