The lanky forward - who replaced Ianis Hagi, got two second-half strikes to neutralise Jonah Osabutey’s first goal for Bernd Hollerbach’s men.
The ex-Midtjylland man has now extended his tally in the Belgian First Division A to four goals in four games as the Smurfs moved to third on the log with 12 points.
Tanzania international Mbwana Samatta was on parade from start to finish, while Stephen Odey made a cameo appearance after coming in for Theo Bongonda.
Onuachu is one of the seven strikers picked by Gernot Rohr’s men for the Singapore friendly against five-time world champions, Brazil on October 13.
The game serves as preparation for November’s Africa Cup of Nations qualifiers against Benin Republic and Lesotho.