Preheat oven to 500°
Pat beef dry with paper towels. Place on baking sheet.
Meanwhile, mix softened butter and mustard together in a small bowl. Spread half of the mixture over one side of the beef with a spatula or spreader. You can use your hands but it gets messy and a lot of butter ends up on your hands and not the beef.
Season the first half with half of the salt and pepper. Add chopped parsley if using.
Turn the beef over. Repeat Steps 3 & 4.
Roast in preheated oven for 25 minutes until thermometer reaches 135° for medium rare. It might take a little longer depending on your oven. If you want rare, roast to 125° and medium 140°.
Remove from oven and rest for approximately 15 minutes. The beef will continue to cook while it rests.
Slice ¼ to ½" thick. For sandwiches, you can slice it thinner if you prefer.