- (3 lbsm trimmed and tied) Ribeye Roast
- (3 tablespoons) Montreal-Style steak seasoning
- (2 Tablespoons) prepared horseradish
- (1/2 teaspoon) Black pepper, freshly ground
- (1/4 teaspoon) Kosher salt
- (1/4 cup) Dijon Mustard
- (2 Tablespoons) Unsalted butter, melted
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.
Meanwhile, whisk sour cream, horseradish, pepper, salt, and 1/4 cup mustard in a small bowl; set aside.
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.
Do Ahead: Sour cream sauce can be made 1 day ahead. Transfer to an airtight container and chill.