Planet of Hats
Donor
Several groups that we nominally call "Mongols" now thanks to ol' Temujin had nominally adopted Nestorian Christianity. The Khereids were fairly Nestorian and were one of the dominant steppe tribes before Temujin. Similarly, you have the Naimans, who were not only somewhat Nestorian, but more numerous and westerly than the Khamag Mongols from which Temujin and his clan emerged.
I'm playing with this a bit in my own TL: The Naimans pushed west and, while not as overwhelming as Temujin, took control of the Tarim Basin and Sogdiana and established a khanate with a Nestorian ruling class with some Buddhists mixed in. If you don't butterfly Temujin, I suppose you could get one of the khanates to become Nestorian simply by positing a different deployment of the Mongol Empire's forces during the phase of conquest: Have a disproportionate number of subject Naimans and Khereids conquer that area and establish a local ruling class which is Nestorian.
I'm playing with this a bit in my own TL: The Naimans pushed west and, while not as overwhelming as Temujin, took control of the Tarim Basin and Sogdiana and established a khanate with a Nestorian ruling class with some Buddhists mixed in. If you don't butterfly Temujin, I suppose you could get one of the khanates to become Nestorian simply by positing a different deployment of the Mongol Empire's forces during the phase of conquest: Have a disproportionate number of subject Naimans and Khereids conquer that area and establish a local ruling class which is Nestorian.