For aquafaba - you can use the drained liquid from canned chickpeas, or make your own if cooking chickpeas from dried. After chickpeas are cooked, remove from water and simmer until the consistency is similar to egg whites.
Using food processor or immersion blender, blend all ingredients except oil until mixture begins to thicken.
While blending, slowly add the oil. This may take 1-2 minutes for mixture to thicken and emulsify.
In a bowl, mash the chickpeas with a fork or potato masher until they are mostly broken.
Fold in mayonnaise, salt, pepper, celery, and red onion.
To assemble, sandwich a scoop of chickpea salad, tomato, and spinach between 2 slices of bread.