| Ingredients | Image |
For the beef: 6 lbs boneless prime rib roast, trimmed and tied 4 tbsp Montreal style steak seasoning, divided For crusting the roast: ¼ cup dijon mustard ¼ cup melted unsalted butter For the cream sauce: 1 cup sour cream 1 cup prepared horseradish ¼ cup dijon mustard 1 tsp ground black pepper ½ tsp kosher salt |
 |
Season roast all over with 2 Tbsp. steak seasoning. Transfer to a wire rack set inside a rimmed baking sheet and let sit at room temperature 1 hour.
Preheat oven to 225°F
Roast beef until an instant-read thermometer inserted into the center registers 120°F, 3 1/2–4 hours (start checking every 5–10 minutes after 3 1/2 hours). Tent with foil and let sit at least 30 minutes and up to 1 hour. Increase oven temperature to 500°F.
Remove string from roast; discard. Stir butter and 1/4 cup mustard in a small bowl. Using a pastry brush, thickly coat roast with mustard-butter sauce. Coat with remaining 2 Tbsp. steak seasoning. Roast beef until a brown crust forms, 5-10 minutes. Transfer to a cutting board and carve. Serve with sour cream sauce alongside.
Whisk sour cream, horseradish, pepper, salt, and 1/4 cup mustard in a small bowl; set aside.
Sour cream sauce can be made 1 day ahead. Transfer to an airtight container and chill.