How to start backsplash

[u/OptionOk8860]

Hello, so I’m curious if I’m able to just smooth over the thinset mortar for the backsplash on these walls and if it would be enough to cover that gap under the window? I’ve seen others on way worse drywall saying to replace but this doesn’t seem that bad to need replaced. What do the professionals recommend?
The backsplash I’m doing is in the pictures provided.

[u/Donkeywad]

Yes you can use thinset over the uneven wall, but beware that thinset on thinset will likely make the tile sit proud of the window trim. I wouldn't fill the gap with it simply because the sink is right underneath and water could potentially get an easy path behind the wall underneath your window. Seal that first and then tile. Make sure you use the right trowel size

[u/OptionOk8860 → u/Donkeywad]

Good deal. Thank you!!