Toast two slices (on one side only). Apply eggless mayo to an un-toasted bread slice. Layer lettuce leaves and tomato slices. Place an un-toasted bread slice over it. Over this slice, place a cheese slice. Layer cucumber slices, lettuce leaves and caramelized onions and cover with the other toasted slice. (To caramelize onions, saute a sliced onion and a tbsp of brown sugar in a 1/2 tbsp of olive oil till brown).