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Melt 2 tablespoons of the butter in a heavy skillet or cast-iron pan over medium heat. Once the foam subsides, add the onion and sauté, stirring frequently, until softened and edges begin to caramelize and turn crispy, about 8 minutes. Add the salt, sugar, and vinegar and stir until sugar dissolves and vinegar is absorbed into the onions, about 2 minutes. Remove from heat and transfer to a bowl. Set aside.
Wipe out the skillet and melt the remaining 1 tablespoon butter in it over medium-low heat, swirling to evenly distribute. Spread ½ teaspoon of the mustard on the inside of each slice of bread and transfer to skillet, with the mustard sides facing up.
Slice the cheese in half into rectangles and lay alternating flavors in shingles on top of the mustard on both slices of bread. Distribute the caramelized onions on top of the cheese and add the pickle slices (if using).
Cover the skillet with a lid and let the cheese steam for 2 minutes. Remove the lid, place one bread slice on top of the other, pressing the cheesy sides together, and cook for 3 minutes. Flip and cook the other side for another 3 minutes, or until browned and crusty. Transfer the sandwich to a cutting board, slice in half, and serve.