Rotate smoothly an object when key is up

I want to make my object rotate smoothly whenever I stop pressing a key (Input.GetKeyUp()). If I understand right, Lerp can not help me with that since the key up event is instantaneous and does not last for some time like Input.GetKey()

So if I'm right, what is the simplest way to do it ? should I use co-routines or there are simplest ways

Thank you