Onuachu put John van den Brom’s men in front with a superb finish after receiving a fine assist from Theo Bongonda in the ninth minute.
Junya Ito doubled Genk’s lead in the 24th minute, firing his effort beyond the reach of goalkeeper Thomas Didillon after he was set up by Bryan Heynen.
The action-packed first-half also saw Hannes van der Bruggen receiving a red card in the 39th minute, having previously been cautioned earlier in the game, to leave Cercle Brugge with one man short.
Both sides failed to find the back of the net in the second half as the Green and Black frustrated every effort from the Luminus Arena outfit to try and increase their lead, despite their numerical disadvantage.
Fashanu: Genk s Onuachu is currently Nigeria s best striker goal.com - get the latest breaking news, showbiz & celebrity photos, sport news & rumours, viral videos and top stories from goal.com Daily Mail and Mail on Sunday newspapers.
Former Wimbledon and Aston Villa attacker John Fashanu has showered encomium on Genk forward Paul Onuachu, describing him as the best Nigerian striker at the moment.
Fashanu: Genk s Onuachu is currently Nigerian best striker goal.com - get the latest breaking news, showbiz & celebrity photos, sport news & rumours, viral videos and top stories from goal.com Daily Mail and Mail on Sunday newspapers.
Former Wimbledon and Aston Villa attacker John Fashanu has showered encomium on Genk forward Paul Onuachu, describing him as the best Nigerian striker at the moment.