Tangy Crockpot Rump Roast

1 3-5 lb rump roast, trimmed
1 package onion soup mix
1 can cranberry sauce, jellied
2 tablespoons butter, softened
2 tablespoons flour

Rinse rump roast and pat dry. Sprinkle onion soup mix in bottom of slow cooker. Place rump roast in next; spoon cranberry sauce around and over roast. Cover and cook on low 10 - 12 hours. Remove roast from crock-pot and allow to rest while you thicken gravy. Turn crock-pot up on high. Blend softened butter and flour into a paste. Whisk it into the gravy. Cover and cook on high for about 10 minutes, until thick. Slice roast into 1/4 inch thick slices and serve with gravy.