Onuachu who featured from the beginning of the game before being substituted on 84 minutes, scored in the 26th minute to double Genk’s lead.
Genk’s other scorers are Theo Bongonda, Jhon Lucumi and Sander Berge.
The win saw Genk sit seventh on 28 points in 16-team league table.
It was Onuachu’s sixth goal in 13 league appearances so far for Genk in the current league season.
He last scored in the league on November 23 in a 2-2 away draw.
His Nigerian teammate and former MFM striker Stephen Odey was An unused substitute in the win against Waasland-Beveren.
Odey has made six league outings for Genk but his yet to find the back of the net this current campaign.
And in the French Ligue 1, Super Eagles winger Moses Simon featured for Nantes in their 1-0 away win against Nimes on Saturday.
Simon played the full 90 minutes in the game which was his 16th in the league and has scored two goals.
The goal scorer for Nantes in the slim win was Imran Louza in the 28th minute, which temporarily took them to fourth on 29 points.