Preheat the waffle iron and spray with nonstick cooking spray; also preheat your oven to 350 degrees F.

In a medium bowl, whisk together the eggs, shredded mozzarella cheese, salt, and pepper until well combined.

Pour about 1/4 of the egg mixture into the waffle iron and cook for 4-5 minutes until crispy; repeat until you have enough chaffles for your sandwiches.

In a separate medium-size bowl, combine the drained tuna, chopped red onion, celery, mayonnaise, and relish, then season with salt and pepper to taste.

Assemble the sandwich by spooning the tuna mixture onto one chaffle, topping with a slice of cheddar cheese, and placing another chaffle on top.

Place the assembled sandwich on a lined baking sheet and bake for 5-7 minutes until the cheese is melted.

Remove from the oven and allow to cool for 3 minutes before serving.