In a large mixing bowl, combine the drained tuna, mayonnaise, sweet pickle relish, Dijon mustard, and apple cider vinegar. Use a fork to break up the tuna and mix everything together until well blended.
Add the finely chopped red onion and diced celery to the bowl. Stir until all the ingredients are evenly incorporated.
Season the mixture with salt, black pepper, and lemon juice. Adjust the seasoning according to your taste preferences.
For added color and flavor, sprinkle the chopped fresh parsley on top (if using).
Cover the tuna salad with a lid or plastic wrap and refrigerate for at least 10 minutes to allow the flavors to meld together. Serve chilled.
This tuna salad can be served on lettuce leaves, in sandwiches, or with crackers for a satisfying snack.
Feel free to customize by adding other ingredients, such as hard-boiled eggs, diced bell peppers, or chopped olives.
Store any leftovers in an airtight container in the refrigerator for up to 3 days.