Combine: In small saucepan, whisk miso paste, mirin, sake, brown sugar, soy sauce, rice vinegar, and honey until smooth.
Add aromatics: Whisk in garlic, ginger, sesame oil, and red pepper flakes if using.
Heat: Place over medium-low heat. Bring to gentle simmer, whisking occasionally.
Simmer: Cook 8-10 minutes, whisking every minute, until sauce thickens to syrup consistency.
Check: Sauce should coat back of spoon but still be pourable. Add water to thin or simmer longer to thicken.
Cool: Remove from heat, cool 5 minutes before using.
Use: Brush on salmon or other proteins before cooking. Reserve some for serving.
Store: Refrigerate in airtight container up to 2 weeks.