Christian Eriksen began the comeback after 17 minutes when he touched in Marcus Rashford's cross but for all United's pressure, Forest still packed a substantial threat.
But they responded in determined fashion to win a Premier League game at Old Trafford from two goals down for only the third time.
In doing so Awoniyi became the first Forest player to score in seven successive Premier League appearances.
Rashford dinked in a free kick and Fernandes headed the ball across to Casemiro who made up for missing an earlier sitter.
Almost inevitably, Fernandes supplied the winner, although it was tough on Forest who had chances to grab a point in 13 nervous minutes of stoppage time.
Persons:
Manchester United's Bruno Fernandes, Lee Smith, Bruno Fernandes, Taiwo Awoniyi, Willy Boly, Christian Eriksen, Marcus Rashford's, Casemiro, Joe Worrall, Fernandes, Rashford, Danilo, Matt Turner, Awoniyi, Andre Onana, United's, Boly, Serge Aurier, Eriksen, Morgan Gibbs, White's, Martyn Herman, Pritha Sarkar
Organizations:
Soccer Football, Premier League, Manchester United, Nottingham Forest, Manchester, Rights, Old, Tottenham Hotspur, Wolverhampton Wanderers, Thomson
Locations:
Old Trafford, Manchester, Britain, Turner