Wash the tomato and romaine leaves. Pat dry. Slice the tomato into 1/4-inch (6 mm) slices. Peel the red onion and slice very thinly into rings; separate rings.

Mix spread

In a small bowl combine mayonnaise, Dijon mustard, salt, and black pepper. Stir until smooth and evenly seasoned. Taste and adjust salt/pepper if needed.

Assemble & finish

Optional toast

If you prefer toasted sandwiches, heat a skillet or griddle over medium-low. Lightly butter one side of 4 bread slices (use the unsalted butter). Place buttered-side down on the hot skillet. Top each with a slice of cheddar cheese to melt briefly while assembling the sandwich. Toast until golden, 1–2 minutes, then remove from heat.

Assemble sandwiches

Lay out the remaining 4 bread slices (or the untoasted sides) on a work surface. Spread about 1 tablespoon of the mayo-mustard mixture evenly onto each of these 4 slices. On each slice, layer 3 oz (about 3 slices) of roasted turkey, 1 slice cheddar (or use the toasted cheese-topped bread), 1 romaine leaf folded if large, 2 tomato slices, and a few rings of red onion. Top with the corresponding second bread slice (buttered side out if you toasted). Press gently to compact.

Finish & serve

If you toasted only the bottom slices, return sandwiches to the skillet for 30–45 seconds per side on medium-low to warm through and lightly crisp the outer bread, if desired. Slice each sandwich in half on a diagonal with a sharp knife. Serve immediately. Leftovers: wrap tightly and refrigerate up to 2 days.