Use a couple of finely sliced spring onions instead of red onion, if you prefer.
Spread each slice of bread with 1 tsp mayonnaise, then top 4 of them with the lettuce leaves and the roast beef. Share the salad between them, then sandwich together with the remaining slices of bread. Cut in half and serve at once, or wrap and keep cool to serve later.
You could also use lean roast pork or ham instead of beef.