Back when they started dating in 2017, the Smash alum, 38, says the hit record producer, 72, threw a Christmas party for her and her friends to try to "woo" her.
It worked — and now, five years later, the couple is happily married with a 20-month-old son, Rennie. To bring things full circle, they will put out their first album together on Nov. 25, and it's only fitting that it's Christmas-themed.
"I think I guilted David," McPhee tells PEOPLE with a laugh over Zoom. "I said something like, 'It's really crazy I'm married to the greatest producer, basically of our lifetime, and I've only had you record me on a few songs.' Then he said, 'OK, well, why don't we just do a Christmas album?'"
The result was Christmas Songs, a seven-track record of reimagined Christmas tunes with a big band sound. The lead single, a version of "Jingle Bell Rock," is set to be released Friday.
"We thought we would just put a couple songs out on Spotify, and then we did this seven-song EP situation and our co-manager started sending it around to a couple labels," McPhee says. "Before we knew it, we had a whole Christmas plan of shows and promotion."
Foster says the process of recording with his wife "was easy because she sings so amazing."
"I got to make the tracks and do things the way I wanted to, then I'd have her come in and plug her vocal in," he says. "It was really fun. There were a couple of songs we were talking about that we didn't know whether she could handle or not, but she handled them like a pro."
When selecting the songs for the project, the couple wanted to stick to the classics.
"With all of the Christmas albums I've done, I've learned that people just don't want to hear new Christmas songs — I mean, other than Mariah [Carey]," says Foster, who has produced Christmas albums for the likes of Michael Bublé, Céline Dion, Josh Groban, Andrea Bocelli, Rod Stewart and Mary J. Blige.
"I wrote a Christmas song called 'Grown-Up Christmas List' which we included on this album. It's not a classic, but it's been covered by a lot of people," he says. "We love the classics — like 'Frosty the Snowman' and 'Winter Wonderland' — because they're so cleverly done."
Morelli Brothers David Foster and Katharine McPhee
Before this project, McPhee and Foster say they always tried to separate their careers from their daily lives.
"People will always ask us at a dinner, 'What music are you listening to right now?' And he and I both just smile and look at each other and say, 'We don't really listen to music,'" McPhee explains. "We don't listen to music in the car, though I do love putting on some jazzy stuff for dinner parties and stuff."
Adds Foster: "It's not our thing to come home and put on music. We have a full life outside of music."
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The couple's lives certainly got a whole lot fuller when they welcomed Rennie in 2021. (Along with Rennie, Foster also has daughters Allison, 52, Amy, 49, Sara, 41, Erin, 40, and Jordan, 36, from previous relationships.)
"He's so much fun," McPhee says. "We're having such a great time, and we're incredibly busy. We wish we weren't traveling quite as much, but we get to take our little guy with us for most of this stuff."
So far, Rennie has traveled internationally to Mexico, Tokyo and Europe and domestically to Hawaii, Omaha, South Dakota, North Dakota and Las Vegas with his parents on their An Intimate Evening with David Foster & Katharine McPhee tour — and McPhee jokes "he won't remember any of it."
Still, Foster says "every day" with Rennie "is an adventure" — and he's already showing his musical talents on the piano and his electronic drum pad.
"We think [he'll be musical], but we don't know," he says. "Kids, strangely, will do something for a while and then it'll amaze you and then they just drop it and they don't do it anymore. So we don't know if the drumming thing is here to stay — I'd rather he have a tennis racket in his hand than a drumstick to tell you the truth. But anyway, if he ends up being a drummer, that would be great too. He loves watching our drummer, JR Robinson."
While McPhee says she'll also sing "little nursery rhymes" to Rennie, she doesn't "go full Céline [Dion] on him."
Ever since she married Foster in 2019, McPhee says she'd "hoped" to have a baby but knew there were "no guarantees."
For Foster, having a new baby was "not something" he thought was going to happen in his 70s, but "I haven't regretted a single day of it."
"I've loved every single day," he says. "It's the standard thing of like, 'Oh, well you were so young when you had your children and you were working day and night, you never saw them.' And there is some truth to that, sadly, for my [older] kids. I was just working a lot. Even though I'm working a lot now, I'll cancel anything just to hang with him. It's just different."
Kevin Mazur/VF20/WireImage David Foster and Katharine McPhee
Though Foster says he and McPhee "know we're an unconventional couple," they are certain they are the right fit for each other.
"With Céline and [late husband] René Angélil, there was a big age difference, and after I was working with them for a long time it just became Céline and René," he says. "It just looked and felt normal. But it was, of course, kind of odd in the beginning. But I think we're over that hump now with five years of being together, and hopefully people just start looking like we belong together because we feel like we do."
Adds McPhee: "Even if they don't, we don't care. We just love our history, how we met and where we are now. Sometimes we look at each other like, 'This is so wild that we're together.'"
The couple first met in 2006 when Foster served as a mentor to McPhee and other contestants on season 5 of American Idol and have "stayed friends through all these years," McPhee says.
As the holidays approach, McPhee and Foster are looking forward to some downtime with their son and family. But, first, they'll do a six-city tour of their Christmas album kicking off Dec. 1 in Stony brook, New York, and running through their Dec. 17 date in Las Vegas.
"I love that we can do everything together. We really do like being together," says Foster, as McPhee adds, "He just said that to me the other night: 'We have so much fun together.' It feels like we've been together 10 years — in a great way."