This recipe is for an actual sauce (not a marinade) and is best used for stir fry. The timing of the corn starch addition is really important. The sauce needs to be cool before the corn starch is added, otherwise the sauce will be too thick to cook with.
1/2 cup soy sauce
1/2 cup mirin (can be found at an asian grocery)
2 tablespoons white sugar
1 big clove garlic and ginger root of equal size, grated
Pinch of red pepper flakes
1/2 teaspoon corn starch
Combine all ingredients except corn starch in a small saucepan and bring to a boil. Boil for ~3 minutes, stirring occasionally. Let cool in the fridge for a few hours. When it’s truly cool (not lukewarm, not room temperature), add the corn starch. Mix thoroughly and let sit in the fridge overnight before using. This recipe makes enough for about two stir frys. Any leftover sauce needs to be stored in the fridge.
Beware: this sauce is super strong. If you use it for a stir fry, I’d recommend serving over plain pasta or rice to break up the flavor. Enjoy!