How do I lay hardwood over a transition in a room that is not level?
Dave Welch may have answered this to a degree But I will put my spin on it.

Break out the mortar bed to expose the subfloor it covers, get either 1/2" plywood which is in reality, slightly less than 1/2" thick and lay it all over the lower floor level bringing it up to withing working distance of the other floor. If that will be too thick, OSB is 7/16" and use that. No need to get anything that is rated as underlayment that will go under a nail down floor. Underlayment is geared more towards vinyl and other thin flooring installs. Screw it down then add your paper and flooring. Re-secure the diagonal subfloor with screws before you add the flooring over the older subfloor.