In a medium size bowl combine, drained tuna, chopped parsley, coriander, green onions, celery, corn, mayonnaise, Dijon mustard, and lemon juice. Mix together and season with salt and pepper.
In a separate bowl, combine the three pieces of cheese and keep aside.
Butter each side of each bread slice. Top with approximately ½ cup of tuna salad, and a handful of three cheese mixture. Top with another slice of bread.
Skillet method: Preheat a skillet to medium heat. Place the sandwich on the skillet and heat for about 2-3 minutes until the cheese starts to melt and it is golden brown. Flip to the other side and cook until the cheese is fully melted and the sandwich is golden brown.