If you need to set trim tile over a surface that is too rough and irregular for the
standard use of thinset, but isn't so far out that you should screed or render to true it up, you can make what I call a "grout sandwich" and set the tile without risk of the tile slumping or the setting material shrinking because it's piled up too much.
Skim coat the substrate and the back of the tile with thinset. Then, apply sanded grout to the back of the piece of tile, judging the quantity of the grout applied as needed to give the desired thickness under the tile and to fill the voids and irregularities in the substrate. Set the piece of tile and line it up, check for lippage, etc. If done correctly, it will stay in place and support the tiles that are set on top as you go up the wall.
This method is good for setting trims, window sills and jambs, ledges, tops of kneewalls, etc. I don't use it for full tiles but only for trims where the amount thinset needed would be too much and the tile would sag and the there'd be a mess.
Here's a picture from a job last summer. We floated the walls (render) but didn't know exactly where to finish the mud beds for the tile trim. (We got it close-within a half-inch or so). When setting the radius trim along the vertical edges of the shower and along to tops of the walls, we used this method to set all the trim. The blue line in the photo shows approximately where the setting bed ended, and the blue X's show where the grout sandwich was used behind the radius trim tile.
I hope this makes sense.
It's a good way to use up that random bag of sanded grout that's leftover from another job.