Put the chickpeas in a large bowl and mash with a fork or potato masher until about half are broken down and the rest stay chunky.
Stir in the Greek yogurt, mayonnaise, lemon juice and Dijon until the chickpeas are evenly coated.
Fold in the celery, red onion, dill, salt and pepper. Taste and add more salt or lemon juice if needed.
Spoon about 1/2 cup of the filling onto 4 toasted bread slices, top with the remaining slices and press gently.
Store leftover filling in an airtight container in the fridge for up to 4 days and stir before using.