Async calls

MainFile.cs

class Session
{
  private List<UseCase> useCaseList;
  
  public void ExecuteSession()
  {
    foreach(var useCase in useCaseList)
    {
      useCase.ExecuteUseCase();
      // Probelm is here. The next use case starts executing before completion of the previous use-case.
      // Because I donot have a control over the Step execution.
      // I am thinking to have AutoResetEvent in my UseCase class and the set from inside the Step class once it is finished execution.
    }
  }
}

class UseCase
{
  private List<Step> stepList;
  
  public void ExecuteUseCase()
  {
     var step = stepList[0];
     step.ExecuteStep();
     // I only need to call first step. Rest of the step execution taken care by Mediator pattern implementation.
     // I am not displaying that here assuming it is not a concerned to my problem statement.
  }

}

class Step
{
  public void ExecuteStep()
  {
    // Do some work.
    // Sometimes this step calls some other method which is async which I donot have control over it and does not 
    // even know that it is async. Its in some other library.
  }

}