Whisk all ingredients until thoroughly combined and smooth.
Taste and adjust seasonings as needed - add more lemon juice for acidity, extra relish for sweetness, or additional mayonnaise if flavors are too strong.
Salt and pepper
For best results, cover and refrigerate for at least 30 minutes before serving to allow flavors to meld together.
If desired, garnish with additional fresh herbs before serving.
Notes
Finely mince the herbs and onion for the best texture - you don't want large chunks in your smooth sauce.
If your sauce tastes too tangy, add a little more mayonnaise to mellow it out.
For extra flavor, try adding a small splash of pickle juice from the relish jar.
Make this sauce ahead of time if possible - the flavors develop and improve after resting in the refrigerator.
Adjust the consistency by adding a small amount of water or extra lemon juice if it's too thick.