The Nigeria international has impressed this season under Bees manager Keith Andrews.
Onyeka told the club's match programme: “People often say that in the Bundesliga, players run a lot compared to the Premier League. When I got there, I felt it. They always want to go forward with every ball.
“In the Premier League, teams know when to calm the game, when to move and when to play some passes before going forward; in Germany, teams just want to go forward at every opportunity.
“It suited me really well; I love to go forward and track back. It was good for my game.”
Onyeka admits a 1-0 win at Borussia Dortmund was a highlight.
“It was my first time playing there,” he said. “The stadium, the yellow wall. I love those games. Dortmund was so loud - the fans are amazing. You could just feel the energy around the stadium.
“Some players are scared of that. Of course, you are nervous going into those games, but as time goes on you settle down.
“We came in as the underdogs, but with positive minds to win the game, and we won 1-0 from a set-piece.
“We beat them home and away. It was a good feeling.”