Combine soy sauce, oyster sauce, brown sugar, mirin, sesame oil, and rice wine vinegar in a small saucepan. Bring to a boil over medium heat, stirring to make sure sugar is dissolved.
Remove from heat and use immediately over prepared udon and vegetables. Or set aside to cool slightly before storing.
Video
Notes
Mushroom flavored soy sauce: You can use regular dark soy sauce.The sauce yields enough to coat 500 grams of udon noodles with 3 cups of chopped vegetables.
Make ahead tips:
This sauce can be refrigerated for up to 5 days and stored in an airtight container.
It can also be frozen for up to 2 months. Cool the sauce completely then transfer to a freezer-safe container. When ready to use, thaw in the refrigerator overnight and use immediately. Warm the sauce over medium-low heat before using.