Whisk all ingredients together in bowl and season with salt and pepper to taste. Cover and refrigerate for at least 30 minutes to allow flavors to meld. (Sauce can be refrigerated for up to 4 days.)

Sprinkle beef with 4 teaspoons (16 grams) kosher salt. Arrange pieces side by side along natural seam, and then tie together at 1 inch intervals to create 1 evenly shaped roast. Wrap roast in plastic, transfer to large plate, and refrigerate at least 24 hours or up to 96 hours.

Using sous vide circulator, heat water to 133°F/56°C in 12-quart container.

Heat oil in 12-inch skillet over medium-high heat until just smoking. Brown roast on all sides, 6 to 8 minutes. Season roast with pepper and place into 2-gallon zipper-lock freezer bag. Seal bag, pressing out as much air as possible. Gently lower bag into prepared water bath until roast is fully submerged, and then clip top corner of bag to side of water bath container, allowing remaining air bubbles to rise to top of bag. Reopen 1 corner of zipper, release remaining air bubbles, and reseal bag. Cover and cook for at least 18 hours or up to 24 hours.

Adjust oven rack to middle position and heat oven to 475°F/246°C. Set wire rack in aluminum foil–lined rimmed baking sheet and spray with vegetable spray. Transfer roast to prepared rack and let rest for 10 to 15 minutes. Pat roast dry with paper towels.

Whisk egg white in bowl until frothy, about 30 seconds. Combine peppercorns, mustard seeds, rosemary, and flake sea salt in shallow dish. Brush roast on all sides with egg white, then coat with peppercorn mixture, pressing to adhere. Return roast to prepared rack and roast until surface is evenly browned and fragrant, 15 to 20 minutes, rotating sheet halfway through roasting.

Transfer roast to carving board and slice into ½-inch-thick slices, removing pieces of twine as you slice, and serve.