Slice the red onion thinly and place in a small, shallow dish. Cover with vinegar and let sit for 10 minutes to help mellow the flavor of the onion.
Add the fresh tulsi leaves to a food processor and pulse until the leaves are finely chopped.
Add the soaked red onion and vinegar and pulse again until well incorporated.
Add the remaining ingredients and blend until smooth.
Taste and adjust salt, vinegar, or spice as needed.
Transfer to a jar and let it sit for at least 30 minutes before serving to allow the flavors to meld.
Store in the refrigerator and use within about 5 days.