if you are using the routing assertion then response should just go back to the client.
like this

The response may well need some transformation though, The variable that is you would need to edit is ${response.mainpart} this will contain the HTML.
You might need to re-write html links etc (you could use the regex expression for example). If you edit variables or combine other variable you can use the template response assertion, but you would want to set the content type to text/html so that the browser knows its HTML.