(Soak the onion slices in water for about 1 hour, then drain.) Toast the bread, coat one side with mayonnaise.
Top one slice of bread with onion, then tomato, then sprinkle with salt and pepper, in that order. Cover with the other slice of bread, mayo side down, to make a sandwich. Cut in half and you're all done.
Story Behind this Recipe
My Canadian host mother made this for me often. I was so surprised that even though there was no meat inside, it was still amazingly delicious!
Don't use butter, just spread with mayonnaise. It tastes best with thinly sliced bread.