How do I convert event callbacks to async tasks in C#?

Question

I'm working with a system that registers methods as event listeners.

A typical pseudocode example is something like this:

public class Foo {

   public void StartOperation(){
      EventSystem.Subscribe<EventFoo>(this, Handler);
      DoSomethingThatWillTriggerAnEvent();
   }   

   void Handler(EventFoo e)
   {
      // Handle the event
   }
}

I would like to be able to wrap this kind of event handling in an async task, so I could do something more like this.

EventFoo e = await DoSomethingThatWillTriggerAnEvent();

What's the best way to go about doing that?