Prepare the crock of a five-quart slow cooker with cooking spray.
Season roast with salt and pepper; lay in the prepared slow cooker crock.
Spread sauerkraut over and around the roast. Pour beer over the sauerkraut. Sprinkle brown sugar evenly over the sauerkraut; top with onion and dot with butter. Season everything with salt and pepper.
Cook on Low for 8 hours.
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