You can not get the html within the @using{...}
statement the way you are doing right now.
The closest thing you can do is use Templated Razor Delegates
public static HelperResult EditableSimpleHtml(this HtmlHelper helper, string key,
Func<string, HelperResult> template)
{
var templateResult = template(null);
//you have your value here that you can return directly
//or you can return HelperResult to write to the response directly
var templateResultHtml = templateResult.ToHtmlString();
return new HelperResult(writer =>
{
templateResult.WriteTo(writer);
});
}
And in your view:
@Html.EditableSimpleHtml("MyKey", @<text>
<h3>Test</h3>
<p>@DateTime.Now</p>
</text>)