How do I rotate y to mouse location with the x rotation staying set?

I'm trying to rotate only the y axis to where the mouse is whereas I want the x rotation to stay 30. The code makes the x rotation 0 and if I make targetRotation.x = 30 that doesn't work either.

     Plane playerPlane = new Plane(Vector3.up, transform.position);
     Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
     float hitDistance = 0.0f;
     if (playerPlane.Raycast(ray, out hitDistance))
     {
         Vector3 targetPoint = ray.GetPoint(hitDistance);
         Quaternion targetRotation = Quaternion.LookRotation(targetPoint - transform.position);
         targetRotation.x = 0;
         targetRotation.z = 0;
         transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, 15f * Time.deltaTime);
     }