Belgian striker Jelle Vossen, signed on loan from Genk in September, had failed to find the net in 12 previous appearances, but was undeniably the star man for Boro.
Vossen had obviously not come up against a defence as charitable as Millwall’s as he hit a remarkable 23-minute treble to tear the Lions to shreds.
As comfortable a victory as you could wish to see stretched Boro’s unbeaten run to seven matches and lifted them up to fourth in the Sky Bet Championship.
Vossen actually thought he had scored his maiden Boro goal in the opening moments as the game got off to a controversial start.
He glanced Grant Leadbitter’s free-kick against the inside of the far post before the ball was clawed out by Lions goalkeeper David Forde.
Vossen and his Boro team-mates were convinced the ball had crossed the line, but with no goal-line technology to refer to referee Mick Russell had to wave play on.
Soon after the 25-year-old was denied by the legs of Forde, but he did not have to wait long for the floodgates to open.
In the 21st minute Vossen raced onto Kenneth Omeruo’s ball over the top and hooked a fantastic finish over Forde and into the back of the net.
Seven minutes later Boro doubled their lead when Adam Reach was allowed the freedom of Millwall’s right flank and cut the ball back for Chelsea loanee Patrick Bamford to slam past Forde for his seventh goal of the season.
It was three by the 35th minute when another long ball, this time from Daniel Ayala, caught out Millwall’s experimental three-man defence again.
Reach had all the time in the world to square the ball for Vossen, who rolled his shot past Forde and into the corner.
And just before half-time Vossen completed his treble, tucking away the rebound after Forde had come out and blocked Bamford’s initial effort.
Millwall did at least improve after the interval – although it was hard to imagine they could have got any worse.
Scott McDonald even bagged a consolation from close range with 12 minutes remaining, only for Boro sub Kike to reply within seconds with a shot in off the post for Boro’s fifth.
It is now just one win in 15 matches for Ian Holloway’s side, who are hovering nervously above the drop zone.
By contrast Boro, for whom Bamford missed enough presentable second-half chances to have secured a hat-trick of his own, have the look of genuine promotion contenders this season.