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In a skillet over medium-low heat, add olive oil. When warm, add onions to pan and cook until softened and deep golden brown, about 30 minutes or more. (If onions begin to dry out or look like they’re going to burn, add a splash of water to the pan and reduce the heat slightly.) Remove from heat and set aside.
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In a bowl, add ground beef, salt, pepper, and worcestershire sauce. Mix to combine and form into four thin, oblong patties (shape patties to match the shape of your bread). In a skillet over medium heat, add patties and cook until browned on both sides. Remove from heat and set aside. Wipe excess grease from the pan.
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Butter one side of each piece of bread. In a skillet over medium heat, layer your sandwiches as so: one slice of bread (buttered side down), one slice of swiss cheese, beef patty, caramelized onions, secret sauce, slice of cheddar cheese, and one slice of bread (buttered side up). Cook until golden brown on the bottom, flip, and cook until golden brown on the opposite side (this should take 1-2 minutes per side).
