Die Werkself had been “lucky” to beat Qarabag in midweek according to Alonso, but their outstanding quality meant such good fortune wasn’t necessary here.
Victor Boniface spurned a pair of good chances inside the opening quarter-hour, but the hosts would be ahead by the midway point of the half through Alejandro Grimaldo.
Called up to the Spain squad this week, the wing-back netted his eighth goal in his last 13 club matches with a spectacular strike, lofting the ball over Frederik Ronnow from range with a vicious finish that left the Union stopper with no chance.
Urs Fischer’s tactics to sit in and contain Leverkusen were backfiring, and Union were fortunate not to be further behind by the break.
Both Jeremie Frimpong and Boniface found themselves denied by Ronnow, but the visiting goalkeeper’s best work came in denying Florian Wirtz, stretching out his left foot to deny the German international who had dribbled straight through Die Eisernen’s defence.
The visitors looked tougher to break down in the early stages of the second period, but that would’ve made Leverkusen’s second goal all the more frustrating for Fischer.
Jonas Hofmann’s contentiously-awarded corner was deadly accurate, and arriving at the back post to steer home was Odilon Kossounou, who netted for the first time in the Bundesliga.
Alonso’s men were now in cruise control, but a third would eventually arrive as the hosts capitalised on some lacklustre Union defending.
A total failure to deal with a delivery into the box allowed Jonathan Tah to peel away at the back post and rifle home a sublime finish, before substitute Nathan Tella rounded off a devastating counter with a tremendous finish at the near post as Die Werkself returned to the Bundesliga summit in comfortable fashion.
They’ve now won all eight of their competitive home matches this term, while they remain unbeaten across seven matches on home soil. Union, meanwhile, are rock bottom in Germany's top-flight having lost nine consecutive Bundesliga matches with their dreadful run showing no sign of ending.
Flashscore Man of the Match: Alejandro Grimaldo (Bayer Leverkusen)
