In a small bowl, combine mayonnaise, finely minced dill pickle, chopped capers, minced shallot (or red onion), lemon juice, chopped parsley, and Dijon mustard.

Stir thoroughly until smooth and homogenous. Taste and season with salt and a pinch of black pepper. Adjust acidity or thickness: add a little more lemon juice if you want brighter flavor, or a touch more mayonnaise if you want a creamier sauce.

Cover and refrigerate while you assemble the sandwich if not using immediately (tartar can be made up to 24 hours ahead).

Sandwich assembly

Prepare bread (optional)

If you prefer a lightly toasted sandwich, spread softened butter on one side of each bread slice and toast in a skillet over medium heat, buttered side down, until golden (about 1–2 minutes). Remove from heat.

Spread a generous layer (about 1 to 1 1/2 tablespoons) of the prepared tartar topping evenly on the inside of each bread slice or on just one slice if you prefer less sauce.

Place one slice of Swiss cheese on the bottom slice of bread (on top of the tartar).

Fold or layer the four slices of ham over the cheese so the meat covers the cheese and the bread evenly.

Top the ham with the two tomato slices, seasoning the tomato lightly with a pinch of salt if desired.

Lay the two lettuce leaves over the tomato (trim any overly large stems so the lettuce sits flat).

Add the remaining slice of Swiss cheese on top of the lettuce if you want cheese both under and over the filling; otherwise skip this step for a lighter sandwich.

Place the top slice of bread (tartar side down if you spread on both slices) to close the sandwich. Press gently to compact the filling.

Cut the sandwich in half crosswise for easy handling. Serve immediately with a pickle spear or chips if desired.