Use these as a side or appetizer, or use Portobello mushrooms in lieu of the button mushrooms for a first course or main. This recipe calls for scallions because they are in the box this week, but if you don’t have them to hand, use white or yellow onions, or even shallots. These can be assembled ahead of time, with the exception of the bread crumb topping which should be added only at the last moment or it will be soggy. When choosing mushrooms, look for those that are still fairly closed and the gills have not darkened yet. Dark gills on button mushrooms are a sign of age, and they are bitter when cooked. If you have your heart set on this dish and the mushrooms all have dark gills, scrape out the gills with a spoon before beginning.