Slice the tomato into 4 even 1/4-inch thick slices. Peel the onion and slice it thinly into rings; separate into individual rings and use about 8–10 thin rings. Rinse and dry the lettuce leaves and pat them with a paper towel. Set produce aside on a plate.

Toast bread (optional)

If you prefer a toasted sandwich, lightly butter one side of each bread slice (about 1/2 tsp per slice) or brush with oil. Heat a skillet over medium heat and toast bread, buttered side down, 1–2 minutes per side until golden. Alternatively, use a toaster. Remove and place on clean surface, buttered/toasted side up.

Mix condiments

On four of the bread slices (these will be the bottom slices), spread 1 tablespoon (1 tbs) mayonnaise evenly on each. On the other four top slices, spread 1/2 teaspoon Dijon mustard on each. If you like more mayo or mustard, adjust to taste.

Build sandwiches

On each bottom slice (mayonnaise side up), layer 2 ounces of sliced turkey evenly (so total 8 oz makes 4 sandwiches). Place one slice of cheese on top of the turkey for each sandwich. Add one leaf of lettuce on top of the cheese, then place one tomato slice on each sandwich. Arrange 1–2 onion rings per sandwich and add 1–2 pickle slices. Sprinkle about 1/8 teaspoon salt and a light pinch (about 1/16 tsp) of freshly ground black pepper across the tomato slices on each sandwich.

Finish and serve

Place the top bread slice (mustard side down) onto each assembled sandwich. If desired, press gently and cut each sandwich in half diagonally. Serve immediately. If transporting, wrap tightly in parchment or sandwich paper to keep ingredients together.