Before popping the question, the actor-musician asked the Aerosmith front-man for permission on March 7 at Chelsea's 25th birthday party. And Steven was more than happy to say yes.
"I fell in love with Jon the minute I met him, and Chelsea loves him over the moon," he told the magazine. "Jon is a great fellow musician and actor. I couldn't be happier."
"It was beautifully old-fashioned," Steven added. "And it meant the world to me when he came to me and asked for her hand in marriage."
Foster, 29, and Chelsea perform together in their blues band BadBad.
Chelsea's half-sister Mia first shared the engagement news on March 28, tweeting: "I'm beyond excited and proud to say that these two turkeys are getting hitched. I love you two to the god damn moon!"