Set the beef in the bottom of the slow cooker. The roast should sit comfortably inside the insert without pressing against the lid.

Stir the canned cranberry sauce and dry onion soup mix together in a bowl, breaking up the sauce as needed. The seasoning should run evenly through the mixture without visible dry pockets.

Spread the cranberry mixture across the roast, then set the butter over the coated meat. The sauce should cover the upper surface, with the butter resting on top.

Scatter the fresh or frozen cranberries into the space around the roast. The berries should sit along the sides, where they can mingle with the cooking juices.

Close the lid and cook on low for 6 hours or high for 4 hours, continuing as needed until the beef yields easily. A fork twisted into the thickest part should separate the fibers without firm resistance.

Pull the tender beef into strands with two forks and turn it through the cooking juices before serving. The shreds should look moist and carry a coating of cranberry-onion sauce.

Add parsley to the finished beef if desired. The green garnish gives the rich, dark sauce a fresh-looking finish.