Slice baguette in half and spread bottom half with mayonnaise. Layer with roast beef, ham and salami. Rinse lettuce, pat dry and slice thinly.
Peel and quarter onion, then slice thinly. Slice olives. Mix together lettuce, onion and olives and spread over salami. Spread the top half of baguette with butter and place on sandwich. Slice into four pieces.