Prepare the Jalapeno: To control the heat, slice the jalapeno in half lengthwise and use a small spoon to scrape out all the seeds and white ribs. This is where most of the heat resides. Finely mince the jalapeno. For a milder sauce, you can use less jalapeno.
Combine the Ingredients: In a medium bowl, combine the mayonnaise, dill relish, minced jalapeno, fresh parsley, fresh dill, and chopped capers.
Mix and Season: Add the fresh lemon juice and black pepper to the mixture. Stir all the ingredients together until they are well combined. Taste the sauce and add salt as needed.
Chill: For the best flavor, cover the bowl and refrigerate the jalapeno tartar sauce for at least 30 minutes before serving. This allows the flavors to meld and the sauce to thicken slightly.
Notes
The sauce can be stored in an airtight container in the refrigerator for up to one week.