Smooth rotation upon button hold press and back

Hello everyone, here is what I want to do:

So I want to rotate the object towards target as long as a key is held down but upon releasing the key I want the object to start rotating back to its original direction and facing.

What I did so far:
void Update () {
movement ();
if (Input.GetButton("Fire1")) {
shoot ();

}

if (Input.GetKey (KeyCode.Q)) {
uTurn ();
}
//Thruster


}

private void uTurn(){

//Turn left
Quaternion newRotation = Quaternion.AngleAxis(180, Vector3.up);
GetComponent<Rigidbody> ().rotation = Quaternion.Lerp (transform.rotation, newRotation, Time.deltaTime * 5f);
}


I get it to turn to where I want it smoothly when I held a key down but when I release the key it just snaps back to the default like faster than lightspeed or something.

 

please use code tags, makes it much easier to read

Code (CSharp):

1. if(pressing button)
2. Lerp (transform.rotation, newRotation)
3. else
4. Lerp (transform.rotation, oldRotation)

 

Thanks a lot @SparrowsNest I acted upon your example and solved it!
And sorry, I will do as you say next time

 

another thing that flew by me, when using rotations you'll want to use Slerp instead of Lerp.