In a mixing bowl combine tuna, mayonnaise, celery, red onion, dill, and cornichons. Season with salt and pepper and stir together.
Spread 1/2 tablespoon butter over each slice of bread and turn two slices butter side down.
Top each butter side down slice of bread with tuna mixture.
Top with sliced tomatoes, shredded cheese and remaining slices of bread.
Heat a skillet over medium heat and melt butter.
Add sandwich and toast for 4 to 5 minutes or until bread is toasted and cheese begins to melt.
Flip sandwich, cover with a lid and continue to toast for an additional 2 to 3 minutes or until bread has toasted and cheese has fully melted.
Uncover, remove from skillet, slice in half and serve.