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1
Build a charcoal fire or heat a gas grill.
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2
Place the ground beef in a large bowl and add the mustard, olive oil, salt, and pepper. Mix gently with a fork to combine, taking care not to compress the ingredients. Shape the meat into 6 (3 1/2 inch) patties of equal size and thickness.
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3
When the grill is medium-hot, brush the grill grate with oil to keep the burgers from sticking. Place the burgers on the grill and cook for 4 minutes. Using a big spatula, turn the burgers and cook for another 3 to 4 minutes, until medium-rare, or cook longer if yo uprefer hamburgers more well done.
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4
Meanwhile, break apart the English muffins and toast the 6 halves cut side down on the grill. Spread each half with mayonnaise and top with a burger and then with a heaping tablespoon of caramelized onions. Serve hot.
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5
Heat the olive oil and butter in a large shallow pot, add the onions and thyme, and toss with the oil. Place the lid on top and cook over medium-low heat for about 10 minutes to sweat the onions.
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6
Remove the lid and continue to cook over medium-low heat, stirring occasionally, for 25 to 30 minutes, until the onions are caramelized and golden brown. If the onions are cooking too fast, lower the heat.
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7
Add the vinegar, salt, and pepper and cook for 2 more minutes, scraping the brown bits from the pan. Season to taste (they should be very highly seasoned).
