In a medium bowl, combine the mayonnaise, sweet pickle relish, chopped dill pickles (or capers), lemon juice, Dijon mustard, parsley, and onion powder.
Stir all ingredients together until they are fully mixed and the sauce is smooth.
Season the sauce with salt and black pepper to your taste.
Cover the bowl and refrigerate the tartar sauce for at least 30 minutes before serving. This allows the flavors to blend.
Taste and adjust seasoning if needed before serving with your seafood.
Notes
For a sharper flavor, substitute the sweet pickle relish with finely chopped cornichons.
If you do not have fresh parsley, you can use 1/2 teaspoon of dried parsley flakes.
You can make this sauce up to three days ahead of time and store it covered in the refrigerator.