Rotate object following mouse movement. Object jumps/ flips

I have a little gnome sitting on a round platform. With a mouse click on the platform I want to rotate the platform around it's Y axis. The angle depends on the position of the mouse.

My implementation is based on the idea of this thread about how to rotate a door according to mouse movement.

Down below is my full script, which is attached to the platform.

The problem: whenever I click on a random point on the platform, the platform will jump/ flip to an unpredictable rotation angle. I tried to prevent that with the rotationStart = transform.rotation. Unfortunately this won't prevent the platform from flipping. What can I do to achieve a smooth rotation?

EDIT: This is the kind of rotation I am trying to achieve:

The left image displays the rotation behavior when applying robertbu's solution. The red cross symbolizes the mouse click position.

I want the rotation behavior shown on the right image.

Current Code

 private var mainCam;
 private var firstHitPos;
 private var rotationPlane : Plane;
 var speed = 0.1;
 
 function Start () {
     mainCam         = GameObject.FindWithTag("MainCamera");
 }
 
 function OnMouseDown(){
         var ray = mainCam.camera.ScreenPointToRay(Input.mousePosition);        
         rotationPlane = new Plane(transform.up, transform.position);        
         var enter : float;
         if(rotationPlane.Raycast(ray , enter)){
             firstHitPos = ray.GetPoint(enter);
         }
 }
 
 
 function OnMouseDrag(){
     var pos = getPlanePos();    
     var tmpPos = Vector3(pos.x, 0, pos.z);    
     
     //var rotationStart= Quaternion.LookRotation(Vector3(firstHitPos.x, 0, firstHitPos.z), transform.up);    
     var rotationStart= transform.rotation;    
     var rotationEnd = Quaternion.LookRotation(tmpPos, transform.up);
     transform.rotation = Quaternion.Lerp ( rotationStart , rotationEnd, Time.time * speed);
 }
 
 function getPlanePos(){
 
     var mousePos = Input.mousePosition;
     var ray = mainCam.camera.ScreenPointToRay(mousePos);
     var mouseWorldPos = mainCam.camera.ScreenToWorldPoint(Input.mousePosition);
     var enter : float;
     
     if(rotationPlane.Raycast(ray , enter)){
         var returnVal = mainCam.transform.position + ray.direction.normalized * enter;        
         return (returnVal); 
     
     }
 }