app.rb

helpers do
  def rest
    @rest ||= RestClient.new('http://foo.org')
  end
end

get '/' do
  rest.put '/foo'
  'hey'
end

app_spec.rb

describe 'My App' do

  setup do
    @rest = mock("rest")
    RestClient.any_instance.expects(:new).return(@rest)
  end

  it 'should do a PUT to "/foo"' do
    @rest.should_receive(:put).with('foo')
    get_it '/'
  end
end