INGREDIENTS
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1 medium sweet yellow onion, sliced
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1 tablespoon extra virgin olive oil
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3/4 pound ground beef (no leaner than 85/15)
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1 teaspoon Worcestershire sauce
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Kosher salt and pepper to taste
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24 slices cocktail size Rye bread
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2 tablespoons butter
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3 ounces sharp cheddar cheese, sliced
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3 ounces swiss cheese, sliced