Make the salt crust:
In a stand mixer (use the paddle attachment) or in a large bowl, combine the salt, egg whites, and water. On medium speed or with a wooden spoon, mix in the flour a little at a time until the dough is firm and feels slightly dry and stiff, like Play-Doh , 2 to 3 minutes in a mixer. Without a mixer, you’ll need to knead the dough by hand. The dough shouldn’t be moist or sticky, and you might not need all 3 cups flour. Cover the bowl with plastic wrap and let rest at room temperature for 2 hours. If making ahead, refrigerate for up to 24 hours.
Prepare the beef:
If the salt crust has been refrigerated, bring it to room temperature. Heat the oven to 400°F. Pat the beef dry with paper towels and season it lightly with salt and pepper. Heat the butter and oil in a heavy skillet over medium-high heat. When the butter has melted and is starting to bubble, add the beef. Sear it until browned on all sides, basting with the butter and oil mixture in the pan, about 2 minutes per side. Transfer to a plate. Discard the excess fat in the skillet.
In a small bowl, beat the egg yolk with 2 Tbs. water and set aside.
Lightly flour a work surface. Roll out the salt crust dough into a rectangle large enough to easily wrap and enclose the beef without stretching or tearing the dough. Set the beef on the dough and lift up each side to enclose the beef as if you’re folding an envelope . Press the seams together and transfer to a heavy baking sheet (a large, flat metal spatula works well for the transfer). If the dough cracks or tears, patch it with a piece of dough pinched from a thicker area. Brush the entire surface of the dough with the egg yolk glaze and sprinkle with a little salt.
Roast on a center rack until an instant-read thermometer inserted through the crust and into the center of the meat registers 125°F for medium rare, 25 to 28 minutes; check early to avoid overcooking. (For rare, cook the beef to 120°F; for medium, cook to 130°F. The temperature will rise a few degrees while the meat rests.) Let rest for 30 minutes in the crust.
Set a mound of the warm potato salad on each plate. Using a paring knife, cut off the top of the salt crust . (You can do this at the table for more drama.) With a fork, gently lift out the beef and set it on a carving board. Slice into four pieces. Set a portion of beef on the potato salad. Spoon some of the vinaigrette over the beef and sprinkle with salt and pepper.
nutrition information (per serving):
without added vinaigrette;
Photo: Scott Phillips