If your meat is not a thick center cut, the cooking time will be less. Letting the roast rest allows the juices to distribute.
Let beef stand at room temperature for 1 hour.
Preheat oven to 425 degrees. Season beef all over with salt and pepper. Whisk together butter and mustards; coat beef all over with mixture. Roast on a parchment-lined baking sheet in the upper third of the oven until an instant-read thermometer inserted into the center reaches 135 degrees (for rare) to 140 degrees (for medium-rare), about 40 minutes (time will vary depending on thickness of cut). Let rest for 15 minutes before slicing.