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Pre-heat grill.
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place the ground beef in a large bowl and add the mustard, olive oil, salt, and pepper.
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mix gently with a fork to combine
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shape the meat into 4 patties of equal size and thickness.
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place the burgers on the grill and cook for 4 minutes.
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turn the burgers and cook for another 3 to 4 minutes, until medium-rare
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Top with cheese if you desire
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break apart the english muffins and toast the halves cut side down on the grill.
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spread each half with mayonnaise and top with a burger and then with a heaping tablespoon of caramelized onions.