The Bees made it two eight minutes later when a half-cleared ball dropped for Marcondes, who fired over the keeper and into the far corner with an unstoppable dipping drive from 20 yards.
The hosts stretched their lead just after the half hour, Marcondes playing a slide-rule pass into the path of Mbeumo, who put the burners on to outpace his marker and coolly slot home in the far corner.
Wednesday battled back after the break, but Brentford weathered the storm and exerted their dominance again in the 73rd minute when Dasilva was played in by Benrahma and slotted under the keeper for four.
Brentford were running riot and some intricate passing around the box slipped Fosu in and he fired an angled drive past Dawson into the far corner to complete the rout.
It was no more than the promotion chasers deserved after a bright start that left the Owls chasing shadows for the first 10 minutes until they got a foothold in the game.
The visitors had their moments though during lulls in the game and half-time could easily have arrived with the scoreline at 6-2.
Jacob Murphy almost caught Bees keeper David Raya out at his near post but the Spaniard deflected his speculative effort wide with a flailing leg.
Steven Fletcher saw a header from a superb Barry Bannan cross loop just over with the score at 2-0, but it was all a little half-hearted from Wednesday, who looked leggy after their midweek FA Cup match against Manchester City.
The Owls appeared to be sharper after a half-time talking-to and former Brentford loan man Kadeem Harris brought spark to the side, testing Raya with a sharp drive at his near post.
Murphy should have done better just after the hour mark but fired a volley straight down the throat of Raya, but the visitors' attacks lacked any real conviction.
With the game heading for stoppage time, Wednesday had arguably their best chance to ruin the Bees' clean sheet, Bannon's free-kick finding Tom Lees, whose header was tipped over by the Bees stopper.
Bannon was the only Wednesday player to emerge with any credit from a one-sided affair which ended with the travelling fans calling for the board's sacking.
The win keeps the pressure on the Championship's top two and leaves Wednesday with just one win from 10.