15mm is rough. Everything I've got is quite a bit taller than that.
There is a way to do this though and keep it clean.
Use a facing tool and cut it down VERY SLOWLY just until you have the clearance.
I did this for 3" tire clearance on a DH bike some years ago.
Don't forget to leave a little room for when going over a bump and the bottom of the tire compressing.
The top of the tire expands a little when this happens.
Also remember tha cutting the top of the steer tube rakes out your head angle a little and cutting the bottom does not.