Place tuna steaks in a glass or plastic dish, you can also use a large Ziploc plastic bag. 

I do not know quantities, as I just combine all ingredients together.  I do use more soy sauce than Worcestershire sauce.  You do not need to use salt, as the soy sauce already has enough.  Sprinkle the following on the steaks – soy sauce, Worcestershire sauce, garlic powder, Old Bay Seasoning, Lawry’s Pinch of Herbs, olive oil (2 tblsp.)  and freshly ground black pepper.  Once you have put all of the above ingredients on the tuna steaks, turn them over so that they get the marinade on both sides.  Clear a space in the dish so that you can place some sliced onions in the marinade.  Leave the steaks to marinate for an hour or more - turning over at least once.  I have left them for an entire day, especially when I was invited out to dinner.  I just put them in the fridge and cook them the next day.