Combine the soy sauce, mirin, sake, sugar, onion, and ginger in a medium-sized saucepan. Bring to a boil over high heat.
Reduce the heat to medium-low and simmer for about 8 minutes. The sauce will thicken and reduce a bit the longer you cook it. (Keep in mind that traditional teriyaki sauce is thin and not especially thick.)
Pour the sauce through a strainer, discarding the solids. Cool and store in an airtight container in the refrigerator for up to 2 weeks.