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1
Heat oil in a 12-inch heavy skillet over medium heat until it shimmers, then cook onions with 1/2 teaspoon salt and 1/4 teaspoon freshly ground black pepper, covered, stirring occasionally, until golden and softened, 15 to 20 minutes.
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2
Reduce heat to medium-low, then add vinegar, scraping up the brown bits on bottom of the skillet, and cook, uncovered, stirring, until liquid evaporates, 1 to 2 minutes. Season to taste with salt and pepper. Transfer to a small bowl and keep warm, covered.
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3
Prepare grill for indirect-heat cooking over medium-hot charcoal (medium heat for gas; see
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4
).
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5
Meanwhile, cut frozen bacon crosswise into eight sections and distribute evenly in bowl of a food processor, then pulse until finely chopped.
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6
In a large bowl, distribute bacon evenly over beef and drizzle mixture with Worcestershire sauce. Sprinkle evenly with 1 teaspoon salt and 1/2 teaspoon freshly ground black pepper.
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7
Combine mixture as gently and thoroughly as possible with your fingers (be careful not to overmix). Form into 4 patties, about 4 inches in diameter and 1 inch thick.
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8
Brush cut sides of buns with butter.
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9
Oil grill rack, then sear patties directly over coals (or lit burner or burners), covered only if using a gas grill, moving to area of grill with no coals (or over shut-off burner) if flare-ups occur, until browned, 2 to 3 minutes on each side.
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10
Move burgers to area with no coals (or over shut-off burner) and grill, covered, turning once, until they reach 155u00b0F on instant-read thermometer, about 4 minutes total. Top with cheese to taste and heat, covered, until it melts and burgers register 160u00b0F, about 1 minute. Transfer burgers to a clean platter.
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11
Grill buns, buttered side down, until grill marks appear, about 1 minute.
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12
Assemble burgers with buns and accompaniments, topping cheese with caramelized onions.
