In a bowl, combine the tuna, onion, pickle, mustard, Sriracha and pepper; mix until incorporated. Slowly add mayo, starting with one tablespoon until somewhat creamy but not sloppy.
Spread tuna mixture evenly on two pieces of bread. Top with tomato and cheese slices. Top with remaining slices of bread and butter the top layer of bread.
Heat a heavy skillet over low heat and place sandwich buttered side down in the skillet. Butter the other side before turning sandwich.
Toast the sandwiches until the cheese is melted and the bread is golden and toasted. Serve warm.