In a medium saucepan, heat sesame oil over medium heat. Add the onions, garlic, and ginger to the oil and cook, stirring frequently, for 5-7 minutes, or until vegetables are soft and aromatic.
Add the remainder of the sauce ingredients to the pan and stir to combine. Bring to a boil, then reduce the heat and simmer for 20 minutes.
Remove from the heat and allow to cool slightly before transferring to a blender. Cover with a lid and blend until the sauce is smooth.
Transfer to a lidded jar and refrigerate. Sauce tastes best after it has settled in the fridge for 24 hours (but is still great fresh). Will keep in the fridge for 1-2 weeks after opening.