Kitchen Tiling onto Uneven Mixed Surface

[Sophieisking]

Hi lovely DIY helpers!

I have done a lot of tiling in my time as a lover of DIY but never onto a surface like this of my sister’s kitchen. Removing the old tiles has left an uneven and very mixed surface. When I’ve finished removing all the old tiles, I just want to tile to where the old tiles were originally.

Please can you let me know whether I can tile straight onto this wall and just bed the tiles with a thicker bed to allow for the deeper depth in different areas or do I need to do something to the wall first?

thank you in advance