Peel the cucumber and slice it thinly. Slice the ham to match the cucumber length.

Drain the oil from the tuna can and mix it with 2 tbsp of chopped onion, mayonnaise, and white pepper.

Prepare the bread sauce by mixing mayonnaise and mustard sauce together.

Trim the crusts off the bread slices and flatten them with a rolling pin.

Place a piece of plastic wrap under each bread slice. Spread the bread sauce thinly over the flattened bread.

Arrange the tuna filling, cucumber slices, and ham neatly on the bread.

Roll the bread tightly like sushi and seal the edges. Let it sit for 30 minutes to set.