Asp.NET MVC Core Authentication?

I'm attempting to set a login system on an asp.net mvc core 1.0 (previously mvc6) that does some custom stuff. In the past I've always set a value in the FormsAuthentication cookie like this on login...

FormsAuthentication.SetAuthCookie(session_id, true);

And then picked out the value out on the request using OnAuthorization in the mvc controller class

protected override void OnAuthorization(AuthorizationContext filterContext)
{
    int session_id = 0;
    if (int.TryParse(User.Identity.Name, out session_id))
    {
        // ... code to lookup user/etc removed for brevity ...
        HttpContext.User = new Principal(user.FullName, "");
    }
}

It appears OnAuthorization is not present any more, and I'm unsure if forms authentication is there either.

How can we do something similar in the new asp.net mvc core stuff?