INGREDIENTS
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3 tablespoons vegetable oil
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2 thinly sliced large onions
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1/4 small onion, finely chopped
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1 pound ground beef chuck (20% fat)
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1 tablespoon ketchup
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1/2 teaspoon garlic powder
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1/4 teaspoon freshly ground black pepper
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1 teaspoon kosher salt
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4 slices seeded rye bread
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4 ounces thinly sliced aged sharp cheddar
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4 ounces thinly sliced Swiss cheese
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4 slices bread
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1 teaspoon mayonnaise
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parchment or waxed paper