One 6-pound sirloin tip roast, preferably grass-fed, tied
1/2 cup prepared horseradish
2 tablespoons kosher salt
2 tablespoons Dijon mustard
2 tablespoons finely chopped parsley
1 tablespoon coarsely ground black pepper
1 tablespoon sugar
1 tablespoon sherry vinegar
Preheat the oven to 375°. Set a rack in a large, deep roasting pan and place the beef roast on the rack.
In a small bowl, blend the horseradish with the salt, Dijon mustard, chopped parsley, ground pepper, sugar and sherry vinegar to form a paste. Slather the paste all over the top and sides of the meat. Roast in the lower third of the oven for about 2 hours, until an instant-read thermometer inserted in the center of the roast registers 125°. Transfer the roast to a cutting board and let rest for at least 20 minutes.
Discard the string and thinly slice the roast beef across the grain. Transfer the meat to a platter and serve.
Serve with mashed potatoes
The unsliced roast beef can be refrigerated for up to 3 days. The sliced roast beef can be wrapped in plastic and refrigerated overnight.