Divide the butter into tablespoons. Spread one on each slice of bread on the designated 'outer' side. Reserve the last tablespoon of butter.
Melt the remaining tablespoon in a cast iron skillet. Saute the onions in the butter until tender, and just starting to caramelize. Remove them from the skillet, and set aside.
Place one buttered slice of bread down into the skillet. Top with one sliced of cheddar cheese, the pickles, the sliced onions, the remaining slice of cheese. Close it with the second slice of bread, butter side up.
Cook the sandwich over medium low heat, for 2-3 minutes or until the bottom side's golden brown & the cheese has begun to melt. Carefully flip the sandwich, and cook another 2-3 minutes until the other side's also golden brown, crisp, and the cheeses are thoroughly melted.
Remove the sandwich from the skillet. Slice & serve.
As written, the recipe is for creating one sandwich. It easily doubles, triples, quadruples etc to feed a hungry crowd.