omg.rb

require ".bundle/environment"
Bundler.setup

require "action_controller/railtie"

class FooController < ActionController::Base
  def bar
    self.response_body = "HELLO"
  end
end

class MyApp < Rails::Application
  config.session_store :disabled

  routes.draw do
    match "/foo" => "foo#bar"
  end
end

run MyApp