In a medium bowl, mix tuna with mayonnaise, pickles, celery, onions, mustard, salt, red pepper flakes and the juice of 1/2 lemon and mix until well incorporated.
In a cast iron skillet, melt a tablespoon of butter ver medium heat. Add 2 slices of bread to the skillet. Layer tuna mixture, a slice of gruyere and cheddar, bacon, and top with bread slices. Once cheese begins to melt, carefully turn the sandwiches over and heat on the other side for about 2 to 3 minutes, or until cheese is fully melted and the sandwich is golden brown. Add more butter to the skillet and repeat with the other remaining bread and mixture.
Serve warm with grapes and a salad.