You can do:

->onClick(Client::dashboard($this->dashboard())->coolMethod())

Let say your Dashboard has an action like this:

class MyDashboard extends Dashboard
{
    protected function actionDoSomething($request, $response)
    {

    }
}

At topMenu you can call MyDashboard action at server by following javascript (MyDashboard is an active dashboard)

->onClick("dashboardAction('doSomething')")

Will be in the docs :)