Routing: The current request for action […] is ambiguous between the following action methods

You can only have a maximum of 2 action methods with the same name on a controller, and in order to do that, 1 must be [HttpPost], and the other must be [HttpGet].

Since both of your methods are GET, you should either rename one of the action methods or move it to a different controller.

Though your 2 Browse methods are valid C# overloads, the MVC action method selector can’t figure out which method to invoke. It will try to match a route to the method (or vice versa), and this algoritm is not strongly-typed.

You can accomplish what you want using custom routes pointing to different action methods:

… in Global.asax

routes.MapRoute( // this route must be declared first, before the one below it
     "StartBrowse",
     "Gallery/Browse/Start/Here",
     new
     {
         controller = "Gallery",
         action = "StartBrowse",
     });

routes.MapRoute(
     "ActualBrowse",
     "Gallery/Browse/{searchterm}",
     new
     {
         controller = "Gallery",
         action = "Browse",
         searchterm = UrlParameter.Optional
     });

… and in the controller…

public ActionResult Browse(string id)
{
    var summaries = /* search using id as search term */
    return View(summaries);
}

public ActionResult StartBrowse()
{
    var summaries = /* default list when nothing entered */
    return View(summaries);
}

You might also be able to keep the action methods named the same in the controller, by applying an [ActionName] attribute to one to distinguish it. Using the same Global.asax as above, your controller would then look like this:

public ActionResult Browse(string id)
{
    var summaries = /* search using id as search term */
    return View(summaries);
}

[ActionName("StartBrowse")]
public ActionResult Browse()
{
    var summaries = /* default list when nothing entered */
    return View(summaries);
}