"It’s the mash-up we’ve all been waiting for...."
INGREDIENTS
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4 medium russet potatoes (8 to 10 ounces each)
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1 pound yellow onions (about 1 large or 2 medium)
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1/2 small bunch fresh thyme (about 10 sprigs), plus more for serving
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1 clove garlic
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2 ounces Gruyère cheese
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4 tablespoons unsalted butter, divided
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1/2 teaspoon kosher salt, plus more for the potatoes
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1/2 teaspoon freshly ground black pepper, plus more for the potatoes
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1 tablespoon all-purpose flour
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1 cup beef broth
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1/2 teaspoon balsamic or sherry vinegar