This would be better written as:

```js
beforeEach(function() {
  this.attachHTMLStub = this.sinon.stub();
  this.View = Marionette.CompositeView.extend({
    template: function() {},
    _attachHTML: this.attachHTMLStub
  });
  this.view = new this.View();
  this.view.render();
});

it('should render according to the custom _attachHTML logic', function() {
  expect(this.attachHTMLStub).to.have.been.calledOnce.and.calledWith(...);
});