Programmatically redirect unauthorized users using Microsoft.Owin.Security.OpenIdConnect

Question

When I first started developing my MVC app (which requires authorization, to selectively render a SPA, which in turn does it's own authorization via ADAL.js) I simply decorated the HomeController's Index method with [Authorize] like thus,

    [Authorize]
    public ActionResult Index(bool ok = false)
    {
        return View();
    }

and everything works fine.

However, sometimes I want users to share URLs with each other. My SPA uses hash-based routing to render various states. As long as both users are logged in already, they can share links with each other and everything magically works ok.

But, if the user clicking a link is yet-to-login, we lose the state communicated in the URL.. everything after the hash.. e.g. if user A sends unauthenticated user B this URL: http://mycorp.com/#/orders/85/edit the HomeController's going to notice user B is unauthenticated, and the middleware is going to kick user B over to our Microsoft login page.. and when he redirects back, the critical #/orders/85/edit portion of the URL in is gone.

My solution was to remove the [Authorize] attribute from the Index action, and instead do something like this:

 public ActionResult Index(bool ok=false)
 {
     if (!User.Identity.IsAuthenticated)
     {
         if (!ok)
             return View("SetCookie_Then_RedirectWith_OK_InQueryString");
         if (ok)
             //how do I do this part?
             return new RedirectResult(Microsoft.Owin.Security.OpenIdConnect.Url);
     }
     else
         //now we're authenticated, 
         //let JS resume by looking for cookie set previously
         return View("Index");
 }

Notice the how do I do this part comment above. That's my question.

+++++++++ FWIW ++++++++++

My app features a Startup.Auth.cs file with the familiar middleware declared...

        app.UseOpenIdConnectAuthentication(
            new OpenIdConnectAuthenticationOptions
            {
             ...blah...
            }
        );

I presume I can do something fancy here in Startup.Auth.cs w/Notification Handlers but, I was hoping to directly call the middleware into action from the Index method.