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Patty Melts Recipe

Name: Patty Melts Recipe
Ingredients: 4 to 6 tablespoons unsalted butter; 2 large onions, peeled and thinly sliced; Kosher salt and freshly ground black pepper to taste; 1 ½ pounds ground beef, at least 20 percent fat; 8 slices seeded rye bread; 8 slices Swiss cheese, approximately 1/2 pound