Instructions :
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In a medium skillet, melt butter over medium-low heat. Add onions and cook until caramelized (about 15-20 minutes), stirring often. Add water as needed to prevent burning. Remove from skillet and tent to keep warm.
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In a large bowl, mix together the ground beef, Worchestershire sauce, garlic powder, salt, and pepper.
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Make 4 square patties (a little larger than the bread). Each patty should be about ¼ inch thick.
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Wipe the skillet and set heat to medium-high. Add the hamburger patties in batches (two at a time) and cook 1-2 minutes per side. Remove patties from the skillet and set aside.
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Meanwhile, in a small bowl, combine the mayonnaise, lemon juice, pepper, garlic, parsley, and ketchup to make the aioli spread.
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Top 4 slices of bread with cheese, then hamburger patty, then onions, and then another slice of cheese, aioli spread, and top with a slice of bread.
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Spread 1 teaspoon of mayonnaise on the top.
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Wipe out skillet and set heat to medium. Add sandwiches to pan, mayo side down in batches (two at a time). Place a weighted pan on top to hold the sandwich firmly in the pan. Cook until golden brown (about 3 minutes). Remove the weighted pan. Spread 1 teaspoon of mayo on the top slice of bread and flip. Cook for 3 additional minutes (or until golden brown). Serve.
