Milan were trailing 1-0 at half-time after Andrea Belotti put Torino ahead at San Siro on Thursday, but Montella's in-form side played some of their best football following the break and Bonaventura crossed for Juraj Kucka to equalise just past the hour-mark.
Just three minutes later, Spanish midfielder Suso played a perfectly-weighted pass to Bonaventura, who executed a superb volley into the net to give Milan the lead and the victory.
Montella, who has seen his men lose just one of their last 10 matches, praised Bonaventura's versatility.
"He always played outside in a 4-4-2, but he can play anywhere," Montella told Rai Sport.
"He's a guy who wants to learn, is a very precious asset, both for Milan and for the Italian national team."
Montella admitted Torino got the better of his side early in the game, but saluted the team spirit that helped deliver the turnaround.
"I saw a game in which we were a bit slow in the first half," he said. "Their goal gave us more courage.
"This approach has been repeated again and, with a great team spirit, we were able to win a great victory."
Milan's progress has been built on a blend of youth and experience, and Montella acknowledged the contribution of 30-year-old defender Gabriel Paletta as well as 20-year-old winger Davide Calabria.
He said of Paletta: "He had a big performance. He is a very useful player for us, with his ability to control the defence. It is no longer a surprise to see him do that.
"So many of the side are also young and grew up in our youth team. I think any winning team in the world must find players in their own nursery.
"We have many interesting guys like Calabria, who is proving to be able to play for Milan."