In your food processor bowl (see Note 5), combine the onion, celery if using, dill, and capers. Pulse repeatedly (don’t continuously blend) until the ingredients are finely chopped.
Add in the chickpeas and pulse again until chickpeas are crumbled into small pieces but not pureed. Transfer the mixture to a large bowl.
To the chickpea mixture, fold in the dill pickle relish, garlic, lemon zest, 1 tablespoon (15 mL) lemon juice, mustard, black pepper to taste, ½ teaspoon kosher salt, and mayo (½ cup / 112g mayo for a creamier, richer mouthfeel; ⅓ cup / 75g for a lighter vibe).
Mix everything well with a spatula until well-combined and creamy. Taste, adding more lemon juice, mustard, salt, or pepper as desired.
For toasted sandwiches (my preference): Lightly toast each slice of bread using a toaster, toaster oven, broiler, or a frying pan. While still warm, rub each slice of bread with the cut side of a halved garlic clove. If desired, spread a thin layer of vegan butter on one slice of bread.
Assemble the sandwiches: Scoop a generous amount of chickpea salad onto each slice of bread. Top with sandwich fixings and sprinkle lightly with a pinch of salt and pepper (if desired, lightly drizzle the veggie toppings with extra virgin olive oil). Top with the remaining slice of bread and slice in half, or serve as open-faced sandwiches).