How to make a gameobject look at(rotate) another gameobject smoothly?

Hi there, I have a trouble with rotating a gameobject.

I want to make my gameobject A looks at gameobject B when I click mouse button, but not in one frame, I want to rotate it smoothly as if person rotate its body.

So I came up with a function 'slerp'. but I can't do well with it. Can you give me a solution?

Here is my code :

 public GameObject target;

 void Update()
 {
     FollowTarget();
 }

 void FollowTarget()
 {
     Quaternion lookOnLook = Quaternion.LookRotation(target.transform.position - transform.position);

     transform.rotation = Quaternion.Slerp(transform.rotation, lookOnLook, Time.deltaTime * speed);
 }

and I put another gameobject B in object A's component variable, which I want A to look at, but I failed.