1. Season the beef with salt and pepper. Divide meat into six 1/4"-thick patties that are slightly wider and longer than the bread.
2. Heat 2 tbsp. oil in a 12" cast-iron skillet over medium-high heat. Add the onions, season with salt and pepper, and cook, stirring occasionally, until softened and browned, about 12 minutes. Transfer the onions to a bowl; wipe out skillet. Working in 3 batches, heat 1 tbsp. oil in skillet over high heat. Add 2 burger patties; cook, flipping once, until well browned, about 4 minutes total. Transfer patties to a plate.
3. Top each of 6 bread slices with some of the onions, a cheese slice, and a burger patty. Top each burger with a cheese slice and a piece of bread. Using a table knife, spread butter over the top and bottom of each sandwich.
4. Heat a 12" nonstick skillet over medium heat. Working in 3 batches, cook sandwiches, flipping once, until golden brown and hot, about 6 minutes.
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|Serving Size: 1 Serving (291g)|
|Recipe Makes: 6 Servings|
|Calories from Fat: 510 (63%)|
|Amt Per Serving||% DV|
|Total Fat 56.6g||75 %|
|Saturated Fat 28.8g||144 %|
|Monounsaturated Fat 19.6g|
|Polyunsanturated Fat 3.2g|
|Cholesterol 175.7mg||54 %|
|Sodium 873.5mg||30 %|
|Potassium 549.8mg||14 %|
|Total Carbohydrate 35.1g||10 %|
|Dietary Fiber 4.3g||17 %|
|Sugars, other 30.7g|
|Protein 40.8g||58 %|
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Calories per serving: 815
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