In a blender or food processor, combine the zucchini, olive oil, vinegar, parsley, cilantro, onion, garlic, red pepper flakes, and salt. Process until smooth, then add water if needed to thin out the sauce to help make it more pourable.
Taste and adjust any seasoning to your liking, then serve! This sauce tastes the best the day you make it, but you can store it in the fridge for up to 4 days in an airtight container. (Freeze the leftovers in an ice cube tray if you don't think you'll use it all by then, so you can easily thaw what you need later.)