England's players rallied around Manchester City defender Onuoha at half-time, but more trouble was ignited on the pitch when Matt Derbyshire completed the 2-0 victory by scoring while a Serbia player was injured.
"He's an outstanding young man, not just as a footballer," said Pearce, who coached Onuoha at City.
"I've got that first hand knowledge of him and he comes from a great family. He's a credit to the English nation."
"He's such a strong character, that's the case with most of our players. I told them that is what is going to happen at international level.
"It's difficult to keep restraint under provocation. You've got to learn that in football."
England needed a win to reach the last four, and Leroy Lita headed them into a fifth-minute lead before the trouble started.
Serbia, fielding a second string as they were already in the semi-finals, created chances but Derbyshire completed victory with 13 minutes remaining, sparking a mass brawl as Filip Rydel was down with an ankle injury.
Pearce continued: "Matt's probably one of the most genuine people I've met, he probably would have kicked the ball out if he saw him."
There was still time for Tom Huddestone to receive a red card for dissent almost immediately after coming on as a substitute.
"I said you must learn by that. It's wrong what he has done," said Pearce.
He added: "We're delighted to be in the semi-final. The positives are we are still undefeated."
UEFA will wait for their delegate to produce a match report before action is considered.
Pearce added: "One individual manager sat here won't make a difference.
"It's the powers that be. They were at the game, they saw what went on and will take action if needs be."
Serbia coach Miroslav Djukic claimed he did not hear the racist chanting.
He said: "Racism is definitely a bad thing, I did not see or hear what was happening as I was concentrating on the game.
"In my career I played against players form all nations and these things are bad but I also have never seen a player who is hurt and carry on to score a goal."
