In a large mixing bowl, add the drained can of tuna. Use a fork to break it into small pieces.
Stir in the red onion, mayonnaise, Dijon mustard, and white wine vinegar.
Add the salt, black pepper, dried dill, and parsley. Mix well until everything is evenly combined.
Cover and refrigerate for at least one hour. For the best flavor, let it sit overnight.
Toast two slices of bread for a little crunch, or leave them soft for a classic feel.
Spread the tuna mixture evenly on one slice of bread.
Top with lettuce, tomato, a slice of cheddar cheese, or sweet pickles if you like.
Place the second slice of bread on top, press gently, and cut in half (diagonal or straight—your choice!).
Serve with a handful of chips, a pickle spear, or fresh veggies for a quick and delicious lunch.