Question: What actress and singer who played a starring role in "Grease" was a special guest during the Mormon Tabernacle Choir's 2015 Christmas shows? 

Context articles: List of Mormon Tabernacle Choir organists
Hallelujah! (Mormon Tabernacle Choir album)
List of Mormon Tabernacle Choir music directors
Mormon Tabernacle Choir &amp; Friends
Men of the Mormon Tabernacle Choir
Ring Christmas Bells (album)
This Is the Christ
Keep Christmas with You
Laura Osnes
Spirit of the Season (album)

This list of Mormon Tabernacle Choir organists includes those organists who have accompanied the Mormon Tabernacle Choir in its performances and who are responsible for the four organs on Temple Square.
 They also perform daily 30 minute recitals on the Tabernacle Organ.
 They also provide the organ performances at General Conference at the Conference Center just across the street from the tabernacle, including those parts of general conference with other choirs besides the Mormon Tabernacle Choir.
 The choir currently has a staff of five professional organists:
Hallelujah!
 was recorded during the Mormon Tabernacle Choir's 2015 Christmas shows in the LDS Conference Center, with special guests Broadway star Laura Osnes, actor Martin Jarvis, and guest soloists from the Metropolitan Opera (Erin Morley, Tamara Mumford, Ben Bliss, Tyler Simpson).
 An album and concert DVD was released on October 7, 2016.
 The album will be broadcast on WMHT-FM on December 11, 2016.
 The recorded concert premiered on PBS on December 19, 2016.
The music director of the Mormon Tabernacle Choir is responsible for musical and creative supervision of the Mormon Tabernacle Choir, the Orchestra at Temple Square, the Temple Square Chorale, and the Bells on Temple Square, which are all official musical organizations within The Church of Jesus Christ of Latter-day Saints (LDS Church).
 Part of a group of 85 Welch converts that immigrated in 1849, John Parry was invited by Brigham Young to organize a choir for the church's next general conference.
Mormon Tabernacle Choir & Friends is a studio album by the Mormon Tabernacle Choir featuring several prominent collaborating artists including Sting, Yo-Yo Ma, David Foster, Amy Grant, James Taylor, Santino Fontana, Renée Fleming, Brian Stokes Mitchell, Sissel, Bryn Terfel, Angela Lansbury, and The King's Singers.
 The album topped the "Billboard" Classical Crossover Albums chart for the week of June 3, 2017, its second week on the chart.
Men of the Mormon Tabernacle Choir is a religious compilation album released by the Mormon Tabernacle Choir.
Ring Christmas Bells was recorded during the Mormon Tabernacle Choir's 2008 Christmas shows in the LDS Conference Center with special guests Brian Stokes Mitchell and Edward Herrmann.
 The album was released on August 31, 2009 along with a concert DVD.
 A recording of the concert aired on PBS in December 2009.
This Is the Christ is a studio album by the Mormon Tabernacle Choir.
 The Mormon Tabernacle Choir and Orchestra at Temple Square combine their talents to create this one-of-a-kind collection of music that bears witness of the Lord and Savior, Jesus Christ.
 Featuring songs never before recorded by the Choir, as well as familiar favorites, the album reached #1 on the Billboard Christian Albums chart on July 2, 2011.
Keep Christmas with You is an album recorded during the Mormon Tabernacle Choir's 2014 Christmas shows in the LDS Conference Center, with special guests actor Santino Fontana ("Frozen"'s Hans) and The Muppets from "Sesame Street".
 The album and concert DVD were released on October 16, 2015.
 The recorded concert premiered on PBS on December 21, 2015.
Laura Ann Osnes (born November 19, 1985) is an American actress and singer known for her work on the Broadway stage.
 She has played starring roles in "Grease" as Sandy, "South Pacific" as Nellie Forbush, "Anything Goes" as Hope Harcourt, and "Bonnie and Clyde" as Bonnie Parker, for which she received a Tony Award nomination for Best Actress in a Musical.
 She also starred in the title role of Rodgers & Hammerstein's "Cinderella" on Broadway, for which she received a Drama Desk Award and her second Tony Award nomination for Best Actress in a Musical.
Spirit of the Season was recorded during the Mormon Tabernacle Choir's 2006 Christmas shows "The Spirit of the Season" with special guest Sissel.
 The album was released in 2007 along with a concert DVD and reached No. 1 on the Billboard Classical chart within five weeks of its release and remained there for nine weeks.
 The album was also nominated for two Grammy Awards, Best Classical Crossover Album and Best Engineered Album - Classical.
 The recorded concert was also broadcast on PBS stations in December 2006 to more than 4 million Americans.