Toast slices of bread on one side, and spread other side with butter. On buttered side of one slice, place lettuce leaf, spread generously with Mayonnaise Dressing; then, in order given, a layer each of thinly sliced, mild onion, sliced tomato, sliced hard-cooked egg. Spread with more dressing, and cover with lettuce. Finish with second slice of toast. Sliced cucumbers may be used instead of onion, if preferred.