Adjust Yaw For A Gliding (Like Superman 64) Script

This is a little bit different of a setup from the question answered elsewhere in the forums.

My character should glide like Superman, rolling and pitching with the joystick, and levelling when the joystick is neutral. (e.g. like this)

In addition, the character should rotate around the global Y-axis.

I am so close, but missing something, and I think I just need a fresh set of eyes. Currently, the pitch and roll behave properly, but I cannot figure out how to add the previous Y-axis rotation to accumulate the rotation:

public void AdjustGlidePitchYawAndRoll(float pitch, float yaw, float roll, float speed)
    Quaternion desiredRotation = Quaternion.Euler(new Vector3(0.0f, yaw, 0.0f)) *                 //I can't figure out how to add the previous yaw properly
                                 Quaternion.Euler(new Vector3(90.0f + pitch, 0.0f, 0.0f)) *       //90 degrees, to make the character prostrate by default, like Superman
                                 Quaternion.Euler(new Vector3(0.0f, roll, 0.0f));
    Quaternion rotationThisFrame = Quaternion.Slerp(transform.rotation, desiredRotation, speed * Time.deltaTime);

    transform.rotation = rotationThisFrame;

This code will only rotate toward the current yaw passed in. How would I get the current yaw so I can increase it each frame?
Simply using transform.eulerAngles.y does not work, because it can change depending on the pitch and roll.

I think what I ultimately want to do is use the current roll as a starting point, but I can only figure out how to start from the identity. :frowning:

Here’s a package that has my code thus far (GlideRotation1.cs is current code). Maybe someone can open it and see the simple fix for it… :-\
transform.eulerAngles.y + yaw in your case