Drain and rinse the kidney beans.

Place the beans in a medium bowl.

Mash the beans with a bean masher or fork until mostly smooth.

Add the finely chopped onion to the bowl.

Add the sweet pickle relish to the bowl.

Add the vegan mayonnaise to the bowl.

Add the Dijon mustard or spicy mustard to the bowl.

Add the fresh ground pepper to the bowl.

Combine all ingredients thoroughly.

Refrigerate the mixture for at least 5 minutes to allow the flavors to blend.

Spread the bean mixture evenly on 8 slices of whole wheat bread.

Add lettuce leaves on top of the bean mixture on 4 slices of bread.

Add sliced tomatoes on top of the lettuce.

Top with the remaining slices of bread to form sandwiches.

Serve immediately.