I have a tilted 3D model. How can I spin it while preserving the angle?

I have a 2D game that uses an isometric angle. However, for one enemy, I have a 3D model.

I have the 3D model tilted to match the isometric angle.

I want the 3D model to "look" at the player, and spin so that it's always looking at the player's character.

In the editor, I can grab the yellow rotation line and it'll spin perfectly around the tilted axis I have set. It's doing some kind of calculation in the editor, because when i look at the transform properties, all 3 rotation values are changing as I "spin" the 3D model around.

Any idea on what I need to do in the code so that it will spin, like it does in the editor when I drag the yellow line?