Sinon sandbox does not mock inner function

I have the following code

export default function  monitoring: Monitoring) {
  async function GET(req: express.Request, res: express.Response, next: express.NextFunction) {
    // Create a custom histogram metric
    const httpRequestTimer = monitoring.getHttpRequestDuration();    
    const end = httpRequestTimer.startTimer();

and I am trying to use sandbox to mock the monitoring class and httpRequestTimer.startTimer();

So I wrote

 beforeEach(() => {
    monitoring = <Monitoring><unknown>({
      getHistagram: () => {},
      getPromClient: () => {},
      getHttpRequestDuration: ()=>{}
    });

    GET = status(monitoring).GET;
  });
it('Should return status call when xxx is not finished and pages are 0', async () => {
  const searchMetadata = {
    details: {},
    id: `111`
  };
  const metadata = {
    result_count: 0
  };
  sandbox.mock(monitoring).expects('getHttpRequestDuration').resolves({
    startTimer: () => {}
  });
  await GET(request, res);
});

As soon as I run it I get

  1) test status API
         Should return status call when xxx is not finished and pages are 0:
     TypeError: httpRequestTimer.startTimer is not a function

Also when I console.log(httpRequestTimer) I see

Promise { { startTimer: [Function: startTimer] } }

Any idea how I can mock this scenario?