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preheat the grill
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form the ground beef into 4 square patties the size of the ciabatta rolls
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season with salt & pepper
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heat 1 tbl olive oil and butter in a pan
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add onions and cook slowly for 10 minutes on low to caramelize, turning as needed
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mix tomato paste, 4 tbl olive oil and 4 tbl balsamic vinegar in a jar with garlic, sugar, salt & pepper
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shake dressing and pour over lightly salted arugula and toss
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slice rolls
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grill burgers, rolls and tomato until cooked
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top with mozzarella cheese and close cover to melt
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top toasted bottom bun with cheeseburger, grilled tomato, caramelized onions, arugula and top bun