Spock - Testing onError Callback Method

I have a service class and a method in it where I am calling another method in case the first method gives back an error. I have made every effort to test this simple scenario but spock test would not cover the onError

For Ex: Lets Assume ServiceA is my service interface class

ServiceAImpl implements ServiceA {

    public Method1 () {
          SomemethodBeingCalled().onError(e->SecondMethodtohandleError())
    }
    
         private SomemethodBeingCalled(){}
    
         private SecondMethodtohandleError(){}
    
}

How do test using Spock so that I can cover the onError(e->SecondMethodtohandleError() block ?