The Nigeria international bagged his 26th goal of the season to inspire Genk to a victory at Luminus Arena.
The 26-year-old has been in blistering form for the Blue and White this season and increased his tally to 26 against Cercle Brugge
The leggy forward was handed his 27th league start and helped Genk continue in the winning ways.
Onuachu put John van den Brom’s men in front with a superb finish after receiving assistance 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.
The Blue and White have moved to the third position on the First Division table after gathering 49 points from 30 games.
The Super Eagles attacker has scored 34 league goals in as many appearances since joining the club in 2019.