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1
Heat the oil in a large skillet over medium heat and add the onions, spreading them into an even layer.
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2
Cook, without stirring, until the edges of the onions are beginning to brown, 5 minutes.
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3
Stir the onions and continue to cook, stirring occasionally, until browned and caramelized, another 20 minutes.
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4
Add the chipotles, garlic, 1/2 teaspoon salt and 1/4 teaspoon pepper.
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5
Cook until fragrant and well combined, 1 to 2 minutes, and remove from the heat.
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6
Preheat the oven broiler.
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7
Divide the meat into 12 portions, about 2 ounces each.
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8
Form a patty around each piece of cheese and flatten to 2 1/2 to 3 inches in diameter.
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9
Put the wrapped patties on a foil-lined baking sheet and sprinkle all the patties with 3/4 teaspoon salt.
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10
Broil until the cheese is melted and the burgers are still a bit pink inside, about 6 minutes.
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11
The burgers will be cooked to at least medium in order to melt the cheese but still stay juicy.
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12
Mix the ketchup, balsamic and 1/4 teaspoon pepper in a small bowl.
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13
Brush the burgers with some of the ketchup sauce and divide the rest among the bottom halves of the buns.
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14
Place a burger on each bottom bun, top with some of the spicy onions and finish with a top bun.
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15
Serve warm.
