"Nutty, gooey Gruyère is even tastier when melted on top of savory roast beef and sweet, fragrant sautéed onion...."
INGREDIENTS
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1 olive oil
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1 onion
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1 thyme leaves
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Kosher salt
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Pepper
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4 rye bread
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1 melted unsalted butter or olive oil
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1 whole-grain mustard
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2 Gruyère cheese
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2 roast beef