My favorite sauce for steak, chicken, or seafood is this teriyaki dip. It's thick and full of flavor.
In a small saucepan over medium-high heat, add the soy sauce, garlic, ginger, onion, brown sugar, and rice wine vinegar. Cook until it boils.
Mix the cornstarch with cold water and pour into the soy sauce mixture. Continue to cook over medium-high heat until the mixture boils, stirring frequently.
Once boiling, turn the heat to low and cook for 5 minutes, stirring frequently.
Take the sauce off of the heat and add to a bowl to serve or a lid-tight container to refriegerate.
If the mixture is too runny, add a little more cornstarch to cold water and pour into the sauce on the stove. Let it boil and then turn down the heat while frequently stirring. Be careful, adding too much cornstarch can make it thicken quicken and you'll have teriyaki jelly.
To thin the sauce out, add more water until you get the desired consistency.