Monk was dismissed after just 26 games as Boro manager in December 2017, but the Owls boss extracted his revenge in devastating fashion as his side bounced back from their midweek Carabao Cup defeat to Everton.
The victory lifts Wednesday to the fringe of the Sky Bet Championship play-off places, while Middlesbrough have now suffered back-to-back league defeats.
Monk was hardly renowned for attacking football during his time at the Riverside, but Wednesday could not have provided him with a better start as he returned to Teesside.
The visitors scored two goals in the opening six minutes, with both owing much to some dreadful Middlesbrough defending.
Just a minute later, and Wednesday were profiting from another Bannan set-piece as they doubled their lead.
The midfielder swung over a free-kick from the right, and having outjumped Dael Fry at the back post, Iorfa powered home a header from close range.
Unsurprisingly, Boro were shellshocked, but Jonathan Woodgate's side briefly hauled themselves back into the game as they scored in the 19th minute.
It was McNair's second goal of the season, but it provided no more than a temporary respite for Middlesbrough, with Wednesday adding two more goals before the break.
Reach, who started his career in Boro's academy, claimed the visitors' third goal, steering a first-time shot into the bottom left-hand corner after Atdhe Nuhui teed him up on the edge of the box.
Reach then turned provider shortly after the half-hour mark, receiving the ball from Kadeem Harris after Marc Bola had been caught horribly out of position from a long goal-kick, and crossing for Fletcher to head home at the back post.
Unsuprisingly, Woodgate made a double change at the break, bringing on Daniel Ayala and Marcus Browne, but his side spurned two golden opportunities to at least give themselves a chance of staging a comeback.
Five minutes later and he was wasting an even better opportunity by shooting into the side-netting after he rounded Wednesday goalkeeper Keiren Westwood in the 18-yard box.