Having recently moved up to England Under-19 duty, the Spurs youth netted his first goal at this age range in just his second cap.
Boothroyd's players are getting in additional match time ahead of the beginning of the qualifying round for next summer's European Championships in Germany, due to start in October.
And after an impressive 3-2 win against the hosts earlier in the international break, the Spurs man netted midway in the second half to level the scores in Zagreb and ensure a positive few days for the young group of Englishmen.
Onomah is one of the most highly rated youngsters at White Hart Lane, having made his Spurs debut as at the age of 17 in the FA Cup tie with Burnley last season. He will hope to earn some extra minutes for Spurs in Europa League and League Cup fixtures this year.
Now 18-years-old, the attacking midfielder was on the bench in the final game of last season's Premier League campaign, and was also very much involved with the first team during both the post and pre season friendlies.
Onomah has some previous when it comes to starring in youth tournaments as well. He was in the England Under-17 squad that won the European Championship in 2014, and the Londoner will surely be looking to repeat the feat at a higher age range, before going on to win a squad role for both Spurs and the more senior England squads.
Tottenham teammate Kyle Walker-Peters also featured in the match.