Bathroom Vanity - Backsplash question

[nicer20]

Home ownership means all sorts of DIY projects around the home. And what project can earn the most credits with SWMBO than a bathroom remodel.

Seeking advice - This is the backsplash on the bathroom vanities that I am remodeling.

I got all the old tiles out and was able to do it without any damage to the cement board underneath (photo 1).

Now time to install new mosaic tiles. Since the cement board is intact and nice & flat, I am thinking of installing the new tiles right on top of it.
But here's the challenge. The old tiles had a curved top which left this void between the top edge of the cement board and the bottom of the mirror. (subsequent Photos)
My question - what material can I use to fill this void to bring it leveled with the cement board so I get a uniform surface to set the mosaic tiles? Shall I use some setting compound or just use thick layer of mastic itself? The gap is about an inch.

Any ideas really appreciated .....

Thanks a lot in advance

[twistsol]

Probably not the answer you want, but if it were me, I'd replace the cement backer board with a piece the proper size after removing all the globs of stuff between the existing backerboard and the mirror.

Anything you use to fill the void will move independently of the backerboard that is there and you run the risk of cracked tiles or grout.
A setting compound can grow mold or mildew and condensation will run off the mirror right into that spot.

[nicer20]

twistsol Thanks for these insights. I understand your points.

But removing globs is turning out very tough. The globs have hardened & stuck to the drywall. I was able to remove similar globs on the adjacent walls (no mirrors) only by hitting them hard. But I am afraid I will crack the mirror because globs are literally touching the mirror base plate :-(

[twistsol]

In that case, I would use a setting type joint compound to fill the gap and would coat it with waterproofing. If it is going to end up behind tile the waterproofing step probably isn't necessary, but after a decade in the Pacific Northwest, I tend to do overkill on the mold and mildew prevention.

More likely, though is that I'd end up trying to chisel off the globs anyway and end up breaking the mirror because I wouldn't have thought it through or asked a question first.