Wash the lettuce and tomato. Pat dry. Slice the tomato into three even slices and separate two clean leaves of lettuce. Peel and slice 3 rings of red onion. If using avocado, slice half into thin strips.

Assemble bread base

If you prefer toasted bread, lightly butter one side of each bread slice and toast buttered-side-down in a skillet over medium heat until golden (about 1–2 minutes per side). Leave untoasted if preferred.

Spread condiments

In a small bowl, mix mayonnaise and Dijon mustard. Spread the mixture evenly on the inside of both slices of bread (about 1/2 tbs on one slice and remaining on the other).

Layer sandwich

Place one slice of bread mayo-side-up. Drizzle olive oil lightly over the turkey slices (or brush with the tsp of oil) and arrange the 4 oz of sliced turkey evenly on the bread. Season turkey with 1/8 tsp black pepper and sprinkle 1/4 tsp salt over the tomato slices to taste. Add the cheddar slice on top of the turkey, then layer the tomato slices, red onion rings, and lettuce leaves. If using avocado or pickles, add them now.

Finish and serve

Top with the second slice of bread, mayo-side-down. Press gently, cut the sandwich diagonally (or in half) with a sharp knife. Serve immediately.

Optional serving

Serve with a side of chips, a small salad, or a handful of pickles. For a warm melt variation, place the assembled sandwich (open-faced) under a preheated broiler for 1–2 minutes until cheese is melted, then close the sandwich and enjoy.