"Dense, flavorful aged beef works best for this pan-roasted steak. If you use unaged beef, plan on shorter cooking times...."
INGREDIENTS
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6 tablespoons unsalted butter, divided
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2 medium Vidalia or Maui onions (about 1 1/4 pounds total), very thinly sliced on a mandoline (about 3 packed cups)
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1/4 cup low-salt chicken stock
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1 tablespoon buttermilk
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Kosher salt
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1 tablespoon vegetable oil
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1 28-ounces dry-aged rib-eye steak (about 2" thick), standing at room temperature for 1 hour
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Coarse sea salt
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Assorted soft herb sprigs (such as tarragon, flat-leaf parsley, and chervil)