I like using avocado oil for high heat cooking, like roasting veggies, because it has a higher smoke point, but olive oil will definitely work for this recipe! (eatingbirdfood.com)
Melt the butter (or your choice of high heat cooking oil)-- saute the onions and the mushrooms in medium high. (delightfultastebuds.com)
Regular butter does contain tiny amounts of sugars and proteins and for this reason it tends to get burned during high heat cooking like frying. (healthline.com)