In a medium bowl, whisk together the peanut butter, soy sauce, rice vinegar, sesame oil, honey, minced garlic, grated ginger, and red pepper flakes (if using) until well combined. The mixture will be thick.
Gradually whisk in the hot water, one tablespoon at a time, until the sauce reaches your desired consistency. It should be pourable but still thick enough to coat.
Taste and adjust seasonings as needed. You may want to add more soy sauce for saltiness, vinegar for tang, or red pepper flakes for heat.
Serve immediately as a dipping sauce, dressing, or marinade.
Notes
Store any leftover peanut sauce in an airtight container in the refrigerator for up to 1 week. It may thicken when chilled; simply whisk in a little warm water or soy sauce to thin it out before serving.