Middlesbrough swept aside Millwall with three goals in the first 20 minutes as Neil Warnock's side continued their fine form at home.
Duncan Watmore netted the opener from Marvin Johnson's cross before Marcus Tavernier fired a superb shot into the bottom corner with his left foot.
Watmore then added his second as he reacted quickest to slot home after George Saville had scuffed his shot.
Millwall had second-half chances through Matt Smith and Ben Thompson but Boro were always comfortable.
Watmore, making only his sixth Middlesbrough appearance after being signed as a free agent last month, was central to the victory.