Combine rinsed white beans, 1 tablespoon mayonnaise and ¼ teaspoon pepper in a medium bowl; mash with a fork until well combined but slightly chunky.
Add ¼ cup celery, 1½ tablespoons relish, 1 tablespoon shallot and 2½ teaspoons mustard; stir to combine.
Spread the bean mixture evenly over 2 toast slices.
Top each with cucumber ribbons, tomato slices, avocado slices and bell pepper slices.
Drizzle each sandwich with ½ teaspoon lemon juice, then top each with 4 lettuce leaves.
Top with the remaining toast slices.
Cut in half and serve.
