Stir together tuna, mayonnaise, and sriracha until well combined.
Spread tuna salad evenly over three slices of bread, and top with pickle slices (if desired). Place a cheese slice on top of pickles, cutting cheese to fit bread without hanging over the edge. Place another slice of bread on top of each sandwich.
Carefully place assembled sandwich into hot waffle iron, close lid, and squeeze closed for a few seconds. Allow to cook until timer goes off or until bread is as toasted as you would like. Remove, cut, and serve. (Repeat for each sandwich.)
If you do not have a waffle iron or panini press, heat a couple teaspoons of olive oil or butter in a skillet over medium-high heat. Cook sandwiches on both sides until browned and heated in the center.