Start by opening the can of tuna and draining the liquid. Place the tuna in a mixing bowl.

Add the mayonnaise and Dijon mustard to the bowl with the tuna. Stir until the mixture is well combined.

Toss in the chopped celery and red onion. Mix everything thoroughly, ensuring each ingredient is well distributed.

Add salt and pepper to taste. Start with a pinch, then add more as needed.

Lay one slice of bread on a clean cutting board. Place a couple of lettuce leaves on the bottom slice, then generously scoop the tuna mixture onto the lettuce.

Top with the second slice of bread.

You can cut the sandwich in half or quarters for easier handling, especially if you’re serving it at a gathering!