Turn a mountain of salt into a protective crust that helps this brisket gently steam in its own juices. The result is a moister hunk of beef that doesn't need any fussing over once in the oven -- and it comes out perfectly seasoned.
Preheat the oven to 250 degrees F. Line a baking sheet with foil.
Mix together the salt and egg whites in a large bowl until completely combined and moist (the mixture should hold together when squeezed in your hand).
Heat the oil in a large skillet over medium-high heat. Sear the brisket, flipping as needed, until a dark crust forms on all sides, 6 minutes to 10 minutes.
Meanwhile, spread some of the salt mixture onto the prepared baking sheet so that it is a 1/2 inch longer and wider than your brisket. Lay the brisket on top and season with pepper. Insert an oven-safe thermometer into the thickest part of the brisket. Press and pack the remaining salt mixture on top of and around the brisket so that it is fully encased, making sure there are no holes or cracks. Cook until the thermometer reaches 170 degrees F, 2 hours 30 minutes to 3 hours. Let rest for 10 minutes.
Meanwhile, stir together the sour cream and horseradish into a small serving bowl and refrigerate until ready to use.
Crack the salt crust open with a something heavy, like a meat mallet. Remove the crust, wipe away any residual salt on the brisket with a paper towel or brush and transfer to a cutting board. Thinly slice the brisket and serve with the horseradish sauce.
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