Tau scored the third goal on his full debut late in the game against the third-division outfit.
The Bafana Bafana star finished off a great move by Royale Union by chipping the ball past the goalkeeper at the Joseph Marien Stadium.
He is spending the current campaign on a season-long loan with Saint-Gilloise from Brighton and Hove Albion in the English Premiership.
Watch his first goal in Belgium below:
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