Ingredients

• 1 (6-pound) boneless prime rib roast, trimmed, tied
• 4 tablespoons store-bought or homemade Montreal-style steak seasoning, divided
• 1 cup sour cream
• 1/4 cup prepared horseradish
• 1 teaspoon freshly ground black pepper
• 1/2 teaspoon kosher salt
• 1/2 cup Dijon mustard, divided
• 1/4 cup (1/2 stick) melted unsalted butter

Instructions

1. 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.
2. 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.
3. Meanwhile, whisk sour cream, horseradish, pepper, salt, and 1/4 cup mustard in a small bowl; set aside.
4. Remove string from roast; discard. Stir butter and remaining 1/4 cup mustard in another 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.
5. Sour cream sauce can be made 1 day ahead. Transfer to an airtight container and chill.