Whisk together the beef stock, brown sugar, soy sauce, balsamic vinegar, and Worcestershire sauce until combined. Pour over the beef, seal the bag, and refrigerate for at least 30 minutes or overnight.
Preheat a broiler to high heat. Remove the beef from the ziplock bag and pat dry. Rub the melted butter and olive oil over it, then season with salt and pepper. Place in a baking dish.
Broil for 15 minutes, flip, and broil for another 15 minutes or until it reaches an internal temperature of 125F.
Remove the roast from the oven, cover it in foil, and let it rest for 10 minutes.
Thinly slice it against the grain.
Notes
TO STORE: Place the leftover beef and pan juices into an airtight container and refrigerate for 3 to 4 days. TO FREEZE: Once the beef cools to room temperature, add it to a freezer-safe container or ziplock bag. Freeze for up to 2 months. Let the leftovers thaw in the fridge before reheating. TO REHEAT: Place the steak slices and juices in a pan on the stove with a teaspoon of oil. Cover and warm over medium-low heat.