Dense, flavorful aged beef works best for this pan-roasted steak. If you use unaged beef, plan on shorter cooking times.
Ingredients
6 tablespoons unsalted butter, divided
2 medium Vidalia or Maui onions (about 1 1/4 pounds total), very thinly sliced on a mandoline (about 3 packed cups)
1/4 cup low-salt chicken stock
1 tablespoon buttermilk
Kosher salt
1 tablespoon vegetable oil
1 28-ounces dry-aged rib-eye steak (about 2" thick), standing at room temperature for 1 hour
Coarse sea salt
Assorted soft herb sprigs (such as tarragon, flat-leaf parsley, and chervil)
Directions
Heat 2 tablespoons butter in a 12" heavy skillet over medium-low heat. Add onions and cook, stirring constantly, until translucent (do not brown), 10-12 minutes. Add stock and 3/4 cup water. Bring to a simmer, reduce heat to low, cover, and continue to simmer until onions are falling apart, about 20 minutes. Uncover and stir until onions are almost dry (do not brown), about 5 minutes. Transfer onions to a blender. Add buttermilk and 1 tablespoon water. Purée until smooth. Season with kosher salt.
Preheat oven to 350°F. Arrange a wire rack inside a rimmed baking sheet. Melt 2 tablespoons butter with oil in a large cast-iron skillet or large heavy skillet over medium-high heat. Season steak generously with kosher salt. Cook until a deep brown crust forms, 3-4 minutes per side and 1-2 minutes on edges. Place on prepared rack. Roast until a thermometer inserted into steak registers 115°F, about 40 minutes. (Steak will carry over to medium-rare.) Let rest for 30 minutes.
Melt remaining 2 tablespoons butter in a large skillet over medium-high heat; cook until butter browns, about 2 minutes. Add steak and cook for 30 seconds per side, allowing steak to absorb butter. Cut steak into 4 slices. Place 1 steak slice on each plate and sprinkle with sea salt. Spoon warm onion purée alongside. Scatter herbs over purée.
