From being paid in burgers to 11-minute hat-tricks against Sunderland, here's all you need to know about Boro's latest signing...
Yannick was born in Lyon, France, and moved to London as a baby.
He is from a footballing family; Lomana and Kazenga LuaLua are his cousins.
Yannick's career began as a 16-year-old with Rushden & Diamonds. He also played with non-league side Hillingdon Borough, where he was paid in burgers!
The 31-year-old spent time with Maltese side Floriana as a youngster before returning to English football with Plymouth Argyle.
Yannick has said that if he hadn’t made it in football, he would have concentrated on his career in carpentry.
While at Plymouth, Yannick spent time on loan with Barnet, where he befriended Albert Adomah. The wing duo would later be reunited at Bristol City, Yannick joining The Robins in 2011 as his ascent up the English footballing pyramid continued.
Yannick returned to London with Crystal Palace in 2012, the club where he would go on to notch up just shy of 150 appearances. He would later be managed there by Neil Warnock.
Yannick’s debut for The Eagles was against Boro, a 2-1 victory for us at the Riverside in August 2012. He was included in the PFA Team of the Season for 2012/13.
The wide man was an unused substitute as Palace won the Play-Off final against Watford in 2013 - but would later score against the same opposition at the same venue in the FA Cup semi-final in 2016. Yannick also played in the FA Cup final, where the Eagles were beaten by Manchester United in extra time.
Yannick made his Premier League debut against Liverpool at Anfield in 2013, and scored his first top-flight goal against future employers Everton a little under a year later.
He became the first Palace player to score a hat-trick in the Premier League when he hit three goals in 11 minutes against Sunderland at the Stadium of Light in April 2015.
Yannick joined Everton in August 2016, for a reported £25m fee. He made his Toffees debut against West Bromwich Albion five days later.
Registering his first assist for The Blues against Sunderland in the September, he was provider again in their 3-1 win over Boro a week later - crossing for Romelu Lukaku to score the third goal.
Yannick had scored once - against Burnley - and made a total of four assists in 13 games before a sustaining a cruciate knee ligament injury in a 1-1 draw with Manchester United in December 2016 that would rule him out for over 12 months. He was back in action, again against West Bromwich, on Boxing Day the following year.
In the 2018/19 season, Yannick spent time on loan with Aston Villa and Belgian side Anderlecht. The following year, he was loaned to Portuguese club Sporting Lisbon.
He represents the country of his parents’ birth, the Democratic Republic of the Congo, at international level, as does Britt Assombalonga.
Yannick made his international debut for The Leopards in World Cup 2014 qualifying, in a 0-0 draw with Libya.
He was selected for the 2015 Africa Cup of Nations squad, and scored against Zambia in the group stage. DR Congo lost out to Ivory Coast at the semi-final stage, but beat hosts Equatorial Guinea, including Emilio Nsue in the third-place play-off.