Rinse the lettuce leaf and pat dry. Thinly slice the tomato and the red onion (if using). Lay tomato slices on a plate and sprinkle lightly with 1/4 tsp salt and a few grinds of black pepper; set aside to mellow for 1–2 minutes.

Measure and arrange the deli meat and cheese so they are ready to assemble. Remove bread from packaging so slices are easy to handle.

Assemble Sandwich

Spread 1 tablespoon mayonnaise evenly over one slice of bread. On the same slice, spread 1 teaspoon Dijon mustard (you may mix the mayo and mustard first if preferred).

Layer the protein: place the 3 oz sliced turkey evenly over the mayo/mustard-covered bread slice.

Add the cheese slice on top of the turkey so it sits directly over the warm proteins (if using warmed meat or toasting later) for better melt.

Place the tomato slices over the cheese. Sprinkle the tomato with a pinch (about 1/8 tsp) of the remaining salt and a grind of black pepper. Add the lettuce leaf and optional onion rings on top.

Top with the second slice of bread to close the sandwich.

Finish (optional: grilled)

If you want a warm, grilled sandwich, spread the softened butter evenly on the outside faces of both bread slices. Heat a nonstick skillet over medium heat. Place the sandwich in the skillet and cook 2–3 minutes per side, pressing gently with a spatula, until bread is golden brown and cheese is melted.

If serving cold, cut the sandwich in half diagonally or down the middle with a sharp knife.

Serve immediately. Taste and add more salt or pepper if desired.