I have been making this for years. My version is, thinly sliced zucchini, in heavy based frying pan, seasoned well with sea salt and freshly ground black pepper, then covered with thinly sliced onion and sliced mushrooms. Seasoned well again, then covered with a tight fitting lid and put on the lowest heat for about 30 minutes while I prepare other things. No need for any oil. I sometimes look at it and turn it over with a fish slice. Try it, it is very good.