The summer signing from Dutch side Fortuna Sittard benefitted from a lucky deflection early in the first half before striking again just after the break for his third goal this week.

Millwall have now claimed 15 of their 17 points at the Den this season, while Middlesbrough are still looking for a first away win since early April.

Leeds United loanee Charlie Cresswell was again omitted from the matchday squad, having been left out of the midweek visit to Rotherham United.

Millwall's Zian Flemming (second right) celebrates after opening the scoring for Millwall. Picture: Rhianna Chadwick/PA Wire.

Boro are also searching for a permanent manager after the sacking of Chris Wilder, with interim boss Leo Percovich unable to build on a gutsy victory over Birmingham on Wednesday.

Having rescued a point away at Rotherham in midweek, it was Dutch playmaker Flemming who gave the hosts the lead in the fifth minute.

His low free-kick clipped the bottom of Middlesbrough’s two-man wall and wrongfooted goalkeeper Zack Steffen, the ball fortuitously nestling in the far corner.

And the 24-year-old perhaps should have made it two a couple of minutes later but could not keep his header down from 10 yards out.

Millwall's Andreas Voglsammer takes a shot at goal during the Sky Bet Championship match at The Den, London. Picture: Rhianna Chadwick/PA Wire.

Middlesbrough were showing signs of improvement midway through the first half, culminating in Matt Crooks blazing a poorly-cleared ball over from inside the area.

And striker Rodrigo Muniz was left inexplicably free at the back post at a corner shortly before the break but he too could not hit the target.

Millwall had a golden chance to double their lead on the stroke of half-time as Tom Bradshaw followed up when Steffen failed to deal with a long-range Andreas Voglsammer effort.

But the American made immediate amends, stretching to deny Bradshaw as the striker looked certain to slot home.

The home side started the second half even more quickly than the first, with Flemming smashing in Jake Cooper’s knockdown at the back post in the 49th minute.

Middlesbrough’s Isaiah Jones, returning from a one-match suspension, looked to force the issue with a mazy run into the area.

The 23-year-old went down under a challenge, but referee Matthew Donohue waved away his penalty claim.

Middlesbrough fans broke into an ironic rendition of ‘We’ve had a shot’ in the 85th minute when substitute Matthew Hoppe fired from distance, but it failed to trouble George Long.

A minute later Hoppe had another effort on target, heading low from a corner.

Long this time did well to turn the ball behind at his near post as Boro failed to find a way back into the game.

Leeds United loanee dropped again as Millwall beat Middlesbrough