In a medium bowl, combine the chopped eggs, drained tuna, and chopped celery. Stir in 1 1/2 tablespoons mayonnaise, the Dijon mustard, celery seed, and red onion.
If preferred, add more mayo to reach your desired consistency. Season with salt and pepper to taste.
Spread the tuna mixture evenly on two slices of whole-wheat bread. Place another slice of bread on top of each to create two sandwiches. Enjoy!