Rub pork roast all over with onion soup mix and place roast in the slow cooker. Pour cranberry sauce over roast.
Cook on Low for 8 to 10 hours. Remove roast to a platter, reserving juices in the slow cooker.
Melt butter in a saucepan over low heat. Whisk flour into melted butter to form a paste. Pour reserved juices from the slow cooker into the butter mixture, whisking constantly, until gravy lightens in color and thickens, about 5 minutes. Season with salt and black pepper to taste. Serve gravy alongside roast.