Intercepting IBM's Watson Botkit middle-ware for testing

My project uses the botkit-middleware-watson to talk to IBM Watson. Now I need to make a mock server for this Watson service.

This is just one line of code which calls Watson's API.

await watsonMiddleware.sendToWatsonAsync(bot, msg, null);

My question is, how can I intercept the API call and re-direct the call to my Watson Mock-Server?