Open the can of salmon and drain the liquid.
In a mixing bowl, add the drained salmon.
Add mayonnaise, Dijon mustard, and lemon juice to the bowl.
Mix thoroughly until well combined.
Fold in chopped celery and red onion.
Season with salt and pepper to taste.
Toast the bread slices if desired.
Spread the salmon mixture onto two slices of bread.
Top with lettuce and any optional toppings.
Close the sandwiches with the remaining bread slices and serve.