2. Add to a blender: the chopped onion, chopped ginger, 1/3 cup tamari, 3 tablespoons rice vinegar, the lemon zest and lemon juice, and 2 teaspoons brown sugar (optional).
4. Taste the sauce and, if desired, add the optional 2 teaspoons brown sugar (if you did not add it in step 2) and blend briefly to combine.