Place the bread slices on a clean surface. Spread mayonnaise evenly on one slice, making sure to cover all edges. Spread mustard on the second slice in a thin, even layer.

Add the roast beef slices to the mayonnaise side. Gently fold each slice as you place it down to create soft layers and even coverage across the bread.

Place the cheddar cheese slices on top of the roast beef. Adjust slightly so the cheese covers most of the meat for balanced flavor in each bite.

Add lettuce leaves, followed by tomato slices and onion if using. Spread the vegetables evenly so the sandwich stays balanced. Sprinkle a small amount of salt and black pepper on top.

Place the second slice of bread on top with the mustard side facing down. Press gently to hold the sandwich together without flattening it too much.

Use a sharp knife to cut the sandwich in half. Serve immediately for the best texture and flavor.