Now, I'm not so sure about authentic, but it's still a decent sauce. I've been making batches of this for years, and it's great on steak, chicken, vegetables... whatever. I use it as a marinade, as a dipping sauce, and as a flavor enhancer when I'm cooking. It's sweet and salty. And pretty cheap to make.
The recipe makes a lot, but it also stores well. Before I ever gave a second thought to this spoiling, I simply made it and stored it in a cabinet. For months on end. I figured that since all the ingredients were shelf-stable, the cooked product also was. If you make it with fresh ginger and garlic, it might be a tad less shelf-stable, so I'll leave that up to you.
There are plenty of commercial teriyaki sauces on the market now, unlike when I was given this recipe. But the nice thing about this one is that you can tweak it to your liking. I've been meaning to make it with brown sugar instead of white, just for a change. And you can certainly add more spices.
Or just make this basic recipe and tweak small amounts of the sauce as you use it in recipes.
I haven't made this for a while, but I had enough on hand to marinate some ribeyes destined for the grill.
1 cup hot water
1 cup sugar
1 cup sherry
1 cup soy sauce
1 teaspoon powdered ginger
1 teaspoon garlic powder
Dissolve sugar in hot water in a saucepan. Add the rest of the ingredients and simmer for an hour.