Mix onion, mayonnaise mixture, and mustard in a small bowl; set aside.
While toasting bread, prepare mozzarella, tomatoes, and lettuce.
Spread a portion of mayonnaise on each piece of toast and top with lettuce. Top 4 of the toast slices with tomatoes, cheese, and pita chips; cap with remaining 4 toast slices. Halve and serve.
Recipe by Three Many Cooks at https://threemanycooks.com/recipes/salads-and-sides/meatless-b-l-t/