Turn a large 6-quart slow cooker on low heat. Add butter to the slow cooker and let it melt. Meanwhile, peel the onions. Slice each onion into ½-inch wedges. Throw the sliced onions into the crock and add the Worcestershire sauce. Season with 1 tablespoon salt and 1 teaspoon ground black pepper. Add the brown sugar if needed.Cover and slow cook the onions for 12–14 hours. Stir once or twice if you happen to be around. Once the onions are dark, soft, and sweet, cool to room temperature. Store in airtight containers and refrigerate or freeze until ready to use.