Butter one side of each slice of bread with 1 tablespoon of butter. Melt the remaining tablespoon of butter in a skillet over medium heat. Place one bread slice butter-side-down into the skillet, and top with the Cheddar cheese, pickle slices, and onion. Place the remaining slice of bread butter-side-up onto the sandwich. Cook until crispy and golden brown on both sides, 5 to 7 minutes.